Rebarreling the SKS

[file]

I had an idea based on something I saw on Weaponeer not long ago. It will remain theoretical only until I can afford to do this.

Basically taking an SKS and rebarreling it to a better caliber.


Now the SKS is a very good rifle, sturdy and reasonably accurate. It's only major drawbacks are low mag capacity(easily remedied with a dremel and other tools to make it work with AK mags) and poor caliber.

7.62x39mm is not a very good performer. It's mediocre in all aspects. Some calibers(like 6.5 Grendel) seem like they'd be a much better thing for it. Most of the accuracy problems and limited range are from the caliber the SKS uses. So why not replace the barrel with one of a better caliber and modify the mag slightly to accommodate the slightly longer rounds?


On the Yugo SKS, the barrel is threaded in and has flats on it for a wrench. There are companies out there that produce custom 6.5 Grendel barrels. Someone with a lathe could do the same thing from a barrel blank. It's just a matter of copying the barrel, or possibly changing the length of it for better performance.

The magazine is just a hair too short, but that can be changed by either making a new one or bending the front part of it out(I think).

The spring in the gas system might need to be changed out as well, but I'm not sure.


What do y'all think?

[tomu]

Just sapping the barrel will not do the trick, there have to be changes made to the system especially the bolt (boltface, exctractor, bolt travel distance).

E. g. the rim diameter of the 6.5 Grendel is smaller than that of the 7.62x39 and it's overall lenght is 1.7 mm longer.

[file]

I didn't know if the bolt face would have had to be modified.

I know about the overall length, that's why the mag needs to be modified. Aside from that, there is enough space in the stroke of the action to accommodate it. The only major issue is the bolt face.

I know there have been rebarrelings in 5.56, that required a new bolt(obviously) and those function pretty well.

[486]

.50 Beowulf has the same rim size as 7.62x39, and with slight magazine modifications it should work, or make an AR-15 magazine conversion because the standard magazine would only hold 2-3 rounds. You will also need to mess with the gas port size, but that is relatively easy. It seems the OAL [over all length] is just fine, at 2.19 for 7.62x39, and 2.110 [for most loads] for the .50 Beowulf.

[Hitech_Hillbilly]

File,

How is the 7.62x39 "mediocre in all aspects"? It is currently the most widely used cartridge in the world, and has one of the better reputations of in use calibers. Are you basing your opinion off of it's effectiveness against personal armor? Off of reliability as a man stopping round? It's reliability?

While I am all for finding a better round, not sure if I would be worried about the 7.62x39. i would be more interested in switching calibers if I was shooting a 5.56 or similar varmint type round.

The SKS also does not lend itself to a caliber change as readily as some other platforms. Have you thought of trying an AR-15 style rifle, or maybe going for an AK-74 style if you prefer that type of platform. Not saying you can't re-caliber, but you need to take the position and size of the gas port into account on the SKS. You may end up having to machine several parts from scratch to get something like what you describe to work.

[file]

The 7.62x39 is only used by the Czech Republic anymore if I remember right(in their VZ-58 rifle). It does not lend itself well to accuracy due to it's rainbow trajectory. It is also not a very good manstopper compared to other modern rounds(it does not tumble, expand or fragment in the body). It pales in comparison(in man stopping ability and raw tissue damage) to the 5.56 or the 5.45.

The SKS does not lend itself as easily to caliber change as the AR platform, but I have a "thing" for it. It is one of the few rifles out there that "fits me". It's also cheap compared to the AR or even the AK(the days of the 380$ AK are gone).

486 has a good idea though. If the pressures aren't too much for the system to handle, .50 Beowulf might be worth considering.

The purpose for such a project is to make the SKS better suited to modern use.

[tomu]

As you can see the extractor of the SKS is a semi circle enveloping the cartridge rim, which fascilitates a secure grip as well as guidance of the cartrodge rim.

Unfortunately the cartridge rim of the 7.62x39 is 11.30 mm and the rim of the 6.5 Grendel is 11.20 mm. This might very well lead to extraction problems, especially with semiautomatic fire.

I don't have access to SKS right now, but you might get away with just using a modified extractor as if IIRC the bolt face itself is plane. It might be even as simple as soldering a steel shim to the curvature of the extractor.

Anway the bolt of the SKS is a straightforward milling job so any good machinist with a mill can duplicate one.

[file]

Even without a mill(I have none), an SKS bolt wouldn't be hard to fab. I think that it might be possible to fabricate a new extractor or mod the original one.

As was mentioned before about the .50 beowulf, I did some looking up and it's rim is supposed to be compatible with both the 6.5 Grendel and the 7.62x39, so maybe it won't be as much of a problem for the 6.5 to extract. It's something that would be worth seeing how good a grip it gets is.

Unfortunately I do not have a piece of 6.5 brass or I'd see right now.

[Hitech_Hillbilly]

File,

Hate to disagree, but the 5.56 is not known as a manstopper round. That is why SOCOM is helped in the development of the 6.8 SPC and tried to get it approved. I happen to work with someone who just got back from Iraq, and has also seen combat in Bosnia, and he said the M14 in 7.62x51 is making a dramatic comeback due to it's increased stopping power. When he got back from Iraq, he sold his AR-15 and bought a Springfield M1A.

And with the AK-47 being the most widely used Assault rifle in the world (still) I don't believe that your assertion that only the Czech still use it is correct.

[file]

Within it's practical range, the 5.56 is extremely effective. It's problem is that when fired from short barrels it is not able to fragment beyond short ranges. When that happens, it is nothing but an ice pick. You can't expect such a small and light round to be effective when fired from a 11" or 14" barrel.

All the nations which employed the 7.62x39, aside from the Czech Republic(maybe Slovakia, but I doubt it) have switched to superior rounds like 5.45. Even China has a new round.

The AKM-47 is so popular because the Russkies made a ton of them and sold off most of what they had when they switched to the AKM-74. That means that there are a lot of guns out there which are cheap enough for the dirt poor guerrillas of X country to purchase.

But let's not get into a caliber pissing match, because the 7.62x39 vs. 5.56x45 debate has been going on for decades and will go on until they no longer exist.

[Hitech_Hillbilly]

File,

I agree, let's not get into a caliber pissing match. I happen to own rifles in both calibers (AR-15, SKS, and AK-47) and believe each to have it's own pros and cons. But my point is that you also need to provide examples for statements when you make such broad statements as "mediocre in all aspects". Such overly broad statements have no place on a Science based board such as this without any supporting evidence.

How is it mediocre? Do people not die when they are shot with it? Is it notorious for jamming? Is it widely considered inaccurate? If it is mediocre because it is not the main cartridge in official use by it's country of origin, does that also make the same true of the 30-06 and .308? If you would put forth some type of arguable facts, maybe you could support such a conclusion, but for now, you are just appearing uninformed.

Let's change the target of your scorn to something else for a moment. Let's say you had stated that you wanted to learn to make Semtex because C4 is mediocre in all aspects. I'm sure several members of this board would ask you what you based that conclusion on. Now, would I agree that each of the aforementioned explosives has it's own properties which would cause some to prefer one over the other? Of course. But both will blow the shit out of something, so neither are ineffectual nor mediocre. If you would please list what about the 7.62 round causes it to be mediocre in your opinion, maybe I could see from what you are basing your opinion.

And that's not the only problem with your original post. Let's take a look at the basic premise of your original post. You want to re-caliber a SKS. Why? It's current caliber is "mediocre" and "not a good performer". So without more info, I have to speculate that the basis in your logic is flawed. The SKS is a 60+ year old design. It utilizes a tilt bolt locking design, which is not as secure as breach lug locking. You offer no formula or postulates on what cartridge to use, or what the pressures the SKS design is capable of handling. I'm sure it would be "kewl" to mount an Abrams 120 mm main gun on a WWII era Sherman tank, but if you don't understand the pressures and forces involved, you're going to end up destroying something other than the intended target. Same scenario here.

What is the pressure curve of the .50 Beowulf? While I do believe that the action of an SKS could handle that cartridge, what adjustments need to be made to the gas system? While it is possible to re-barrel or re-caliber the Simonov design, parts are not in abundance. How much will it cost to do the work you are talking about? Would it be a better decision financially to make some other type of weapon for which the caliber you want is already designed for? Would it be easier from an engineering standpoint to modify a different base weapon?

Most improvised weapons are made for low pressure calibers. If they call for high pressure calibers, they are normally some type of single shot design. The mathematics for semi-auto weapon systems are complex. Of the equation E = 1/2mv2 for kinetic energy, only about 30% of the energy gets transfered to the actual bullet. Almost 20% of the energy gets transfered to gas pressure, with the rest going to heat (case, barrel, gas, and bullet). And this total energy is not transfered in a flat line. Each caliber and powder generate their own energy curve. And those curves can change based on cartridge material and bullet weight. And that is just some of the variables.

Just because the forum has added a firearms section doesn't mean that it's high standards are completely thrown out the window. Provide some basis for your arguments, some facts, or at least some arguable conclusions. Most of your other posts do include such facts, plus some logical deductions. All I'm asking is that you do a little research and come up with some supportable reasons for your postulate. This is a Science based forum (whether that be chemistry, mechanical, geopolitical, etc), after all.

[file]

I did do some looking into the cost of such a build. It comes out to 500$ for the 6.5. I haven't run the costs of doing one in .50 Beowulf yet though. That would put the gun, complete, at just under what a cheap AR would cost.

As for dealing with chamber pressure, I was unable to find the max pressure of the .50, but for 6.5 it's 50,000 psi, which puts it very close to the 7.62x39. The maximum load pressure is 52,080. So it should be more than able to handle the pressure given that most loads for the calibers are lower than that max.

Why do I feel that the 7.62x39 is mediocre? It does not fragment, expand or tumble(only FMJ will be considered in this example, because it's obvious what soft point and hollow point will do) while inside the body. It does not create a large permanent or temporary cavity compared to other rounds. The round also tends to overpenetrate unless used with HP or SP. It's saving grace is that it feeds very well and extracts very well due to it's case design. Yes, such a round can blow the shit out of someone. But other calibers can do it better(which is my premise for such a build).

[Hitech_Hillbilly]

File,

Thank you. With that, I can at least see what you are basing your opinion off of. While I don't believe I necessarily agree with all of your conclusions on the various rounds, we can agree to disagree. Your thought process is defined and will allow others to make informed judgments based off of the basis for your conclusion.

Your cost estimate appears to be in line with what a competent gunsmith or machinist would charge you, plus cost of materials, and doing some of the work yourself. I agree with your assessment that you are approaching a low end AR-15 variant. It may be easier in the long run to just purchase a low end AR already in the caliber you want if you are willing to settle for a 5.56 or 6.8 SPC. The 6.5 Grendel or .50 Beowulf may be cheaper to do yourself. From what I've seen, these calibers are rather expensive no matter which route you take.

[file]

It would be least expensive to rebarrel an SKS and that would also be turning an obsolete weapon into a fairly effective DM rifle(if done in 6.5). Not perfect, but passable.

One of the things that would be necessary is a modified mag. Right now I'm thinking of a modified tapco 20 rounder adapted to fit. It may be necessary to fabricate an entirely new magazine, but I'd avoid that if possible. However, if it is necessary to do, it would be much better to make one of a design similar to that of the original mag. It's feeding system is very well suited to being self-contained and fixed in place but is also very reliable. It would have to be slightly longer and deeper with a slightly different follower.

It might be possible to just bend the original mag, but that would give me less than ten rounds per.

This will all have to wait until I have the money to afford the build though.

[486]

You could get an ak mag adapter and remove enough material from that to fit a ar mag in. Then get a 6.5 ar mag [in steel] and braze it into the adapter at the proper height for reliable feeding, braze it with the parts out, and reassemble when cool. It would still be removable [if you get the removable adapter/magazine.

[file]

To my knowledge fitting an AK or AR mag would require filing/milling to the receiver(it is possible, just a lot of work). Fitting an AR mag might actually be possible with a little less filing/milling than an AK mag though. I'm not exactly sure.

[486]

They make these detachable mags for the SKS, they seem to be an AK mag with an adapter welded on. They use the original mag pivot pin as a front catch and the "floorplate" release as the rear catch to hold the magazine in a [somewhat] explanatory picture (http://www.surplusbunker.com/images/MAGSKS20BLACK.JPG) of a molded plastic one. So I assume since the AR magazines don't seem much bigger, you just need to take out the whole internal mag to get it to fit. My dad is in a different country so I can't see if it would fit right now, but in a few weeks I could test it out.

Also for some sort of size comparison [maybe useful] 7.62x39 fits in AR mags, just only 5 rounds or so because of the extreme case taper, and I have heard of people getting .223 in AK mags.

[file]

To get an AK mag to fit, you have to change the cut in the receiver for the mag to fit, because it is tapered in the front on the SKS(not actually too different than the low cap mags really) and straight on the AK. I have mine stripped and am looking at it right now.

Luckily the actual hole seems to be large enough to fit the rounds.

[TreverSlyFox]

Well, trying to make an SKS into an SKS M or D model (Detachable AK47 magazines) has been done before, though none has resulted in a satisfactory or safe modification.

The M & D model DO NOT have the same receiver, yes it looks the same but it's NOT. The receiver walls of the M & D model are thicker to accommodate the AK Magazine. Butchered SKS receivers, even done by a competent machinist wind up being VERY THIN especially in the front where the original SKS magazine tapers. And this is where the receiver cracks and I've seen two that have done so after being modified to take AK mags. Granted I've only seen two SKS's modified like this but that pretty much follows the posts on other boards where this has been discussed and done.

As far as the FMJ Military 7.62x39 not being an effective round I have a few friends whose names are on a wall in Washington DC that would disagree with you if they were still alive. And when they took our M-14's away and gave us M-16's mine lasted less than 2 weeks and I finished my tour with an SKS, which accounted for 14 of 17 confirmed kills, none with the M-16 that I know of.

Also don't forget gas operated semi-autos work on "Peak Gas Pressure" which is NOT the same as Muzzle Velocity, but has to do with the "speed" of the powder burn. So unless your magick bullet has the same "burn" your going to be modifying the fixed gas system (unless your using a Yugo) to get the rifle to function properly without FTF or FTE problems, or battering the rifle apart with over pressure.

Either way, it's been tried before, it didn't work. But hey, good luck to you, maybe you'll be the lucky one.

[486]

So unless your magick bullet has the same "burn" your going to be modifying the fixed gas system (unless your using a Yugo) to get the rifle to function properly without FTF or FTE problems, or battering the rifle apart with over pressure.





The gas port has to be re-drilled anyways for the new barrel for 6.5, besides the gas port adjustment should be pretty easy.

[file]

As stated by 486, the gas port adjustment should be pretty easy. The valve comes right out(on the Yugo model, which is what I stated I'd be using) and can be tinkered with as needed. Obviously starting small and going larger as needed to get it to feed and eject right. Actual pressure curve charts would be best, but I haven't been able to find them.

Trevor, what nationality were the SKS's you've seen fail due to being butchered? The Yugo ones are already beefier than the regular ones, which is why I mentioned going with those in my first post. I also would rather not have to modify the receiver, hence my being big on modifying or copying the original mag design.

I'm not here to start a caliber pissing match either.

[TreverSlyFox]

File'

Of the two I saw one was a Chinese Mil-Surp and the other was a Norinco, (Norinco's aren't Mil-Surp but they are Mil Spec). I've never read anyone trying to modify a Yugo as yet. Sorry I missed the part in you post about using a Yugo, my mistake. I've never looked over a Yugo closely, mostly because I never cared for their un-chromed barrel and chamber and the funky grenade launcher on them.

Sorry if I came off as wanting to start a pissing match on 7.62x39. It's just that there is so much BS put out about the round by people that have never used the round in combat. I have and know what it will do within it's design parameters as a Military round coupled with a totally reliable weapon system like the SKS or AK-47.

The terminal ballistics were greatly improved by the Russians with the 8M1 - 8M3 "effect" bullet but came at the time 7.62x39 was being phased out and the 5.45 was being brought in service so it was never fielded as a service round. Now it's only available in Silver Bear Ammo, if you can find it. I stashed away 5,000 rounds of it when it was $85 a 1,000 rnd case several years ago, now if it can be found it's well over $200 a case. I haven't seen any for sale in a year or more. I have the feeling it's gone the way just as the Chinese steel core did, never to be seen again.

I can understand the want of a little more power in a carbine size action, but then I always look at it from a Military stand point. Cost of ammo, availability or if there is already a Military weapon that already achieves the same thing. Which is probably why out of 15 or so weapons I own 10 or 12 are Military and the first ones I'd grab in an emergency. Each one is set up with web gear and a full combat load out of ammo for those Just-in-Case times.

Sorry if I come off rough at times, I'm an old fart and pretty set in my ways and thinking at 60.

[file]

No offense taken.

The Yugo guns are considerably beefed up compared to their Russian or Chinese counterparts. That's because Yugoslavian ammunition was hotter than other standard production ammo. The only AK mag fed modified ones I've seen are Yugos(because they're the cheapest right now).

I haven't been able to find any of the 8M3 stuff, wish I could though. Sounds pretty good if extremely rare.

The primary desire for having the 6.5 would be to give the SKS more legs as well as more power. 6.5 Grendel out of a 20" barrel is good out a very long ways(I forget exactly how far, but out of a 24 it's good past 1000 yards). That coupled with a very good custom barrel would be able to turn the SKS into a modern battle rifle or possibly DM rifle. The SKS has potential for excellence, you have probably seen that yourself, it just needs to have that fulfilled.

To my knowledge, there are few firearms out there in that caliber and those that are out there are expensive. Ammo cost for Wolf Gold(not the best, but still passable) isn't too much more than 7.62x39 per case. Once I get into handloading, that will drop costs even more, especially considering that 6.5x39 can be formed from 7.62x39 brass cases.

[TreverSlyFox]

File,

Your project really piqued my interest and I spent a few hours checking out the 6.5mm Grendel on the web. While the project seems VERY doable in a Yugo SKS there does seem to be a couple problems in the ammo department.

True 6.5mm Grendel is a straight walled case, while 7.62x39mm is a tapered wall case. And to reform 7.62x39mm into 6.5mm Grendel cases is quite a project as a bulge will form just below the shoulder of the 7.62x39mm case when full length resized. While this can be dealt with by “turning”, its one more step added to the process. Plus the new formed case has to be “fire formed” as a final step and you still don’t have a “True” 6.5mm Grendel case.

The 6.5mm Grendel takes a “Small Rifle Primer” while the 7.62x39mm takes a “Large Rifle Primer” which does and will affect performance which seems to be the large rifle primer “blowing” powder and bullet down the bore “before” the powder ignites. Why this happens with a “straight walled” case and not to a “tapered wall” case is beyond me.

So you have a choice to make before you chamber the new barrel. Either chamber it for 6.5mm Grendel or chamber it for 6.5x39mm. The 6.5x39mm is just a necked down 7.62x39mm case and since it’s just necked down you don’t have the problems with full length resizing and the subsequent bulge, turning and fire forming in trying to go to a straight walled Grendel case. The 6.5x39mm suffers a little in the velocity department compared to the true Grendel, but we’re talking less than 100fps out of a 24” barrel so I don’t see it as a real problem.

If you decide to go the true Grendel way you’re looking at buying cases from Alexander Arms or Lapua. Though the Lapua is considered a better case the price difference is about $28 per one hundred cases, AA is about $64 per 100 and Lapua runs about $92 per 100.

If it were me I think I’d just suck up the 100fps velocity difference and go with the 6.5x39mm and neck down 7.62x39mm cases. I think you’d have fewer problems in feeding; no change to the magazine, still feed it from stripper clips and cheap but tough Military cases to work from.

While you’ll never have a “true” MBR since you’re not firing a “full size” military cartridge, I think you’d have a kick ass DMR in a carbine size package with a good set of optics. Midway sells the Choate Weaver-Style Scope Mount for the SKS (item #435-151) though you’ll have to drill and tap the receiver for it. But it appears to be a rock solid steel mount that won’t interfere with disassembly and cleaning from what I can see of it. Midway has it on sale right now at $30 rather than the normal $38.99, so you may want to look into it. With a set of “quick change” rings you could pop off the scope if it took a dump and be back to the original iron sights which you can’t do with a “scout” mount and a LER scope. Though this has disadvantages also.

The scope mount will kill the use of stripper clips for reload. Though you “could” turn the mount around so it off set to the left and retain feeding from stripper clips and add a cheek pad to the stock. Same thing they did with the M-1 Garand Model “C” sniper rifle and having the benefit of retaining the use of the Iron sights without removing the scope.

While I doubt you’ll get the 1.198 group at 600 yards the true Grendel gets with a modified semi-auto SKS. I don’t see any reason you couldn’t get say 3 MOA at 600 yards which should put you into Minute-of-Bad-Guy out to 900 - 1000 yards with 6.5x39mm. Over all from the little investigation I’ve done the project seems doable and kick ass if you stay in the 6.5x39mm and not try to go for a true 6.5mm Grendel package.

The change over to AK-47 mags would be dependent on if the Yugo can handle the cuts to the receiver wall without compromising the strength of the receiver. While this would eliminate the need to feed it from stripper clips and an "off set" scope. It will also not allow you to get as close to the ground in the prone position with a 30 or 40 round mag sticking out the bottom of the rifle. Gain some, lose some so you have to choose what you want and how you'll use the rifle to make the choice.

No matter what you choose please keep us updated on the project I'm very interested in how it turns out.

[file]

Thanks for the suggestion of 6.5x39mm and just using that. As far as a scope mount, if it interferes with stripper clips isn't much of an issue if it's a DM rifle but it would be a plus. I remember seeing a mount out there that was a "reproduction" of the issue scope mount for it(which accommodated the PU scope and PSOP scope), and those did allow for stripper clip use.

6.5x39 is definitely the way to go in the price area, I can't afford to feed something that is $92 a case but I should be able to afford to neck down 7.62 brass.

This is a long term project. It'll be a while before I save up enough to start the build. Should turn out well though.