Lapping a .308 barrel to .311?

[hq]

"...it's a "squeeze bore" gun..." No, it isn't. There's no such thing. You have an early Mini-30(inaccurate and over priced) made before 7.62 x 39 ammo was easy to come by and cheap, Stateside.

.308 leftover barrels from abandoned Ruger XGI project were used to manufacture Mini 30:s until approximately 1994. I'm not quite sure what you'd like to call the tapered throat that was machined to mate .311 7.62x39 chamber to the .308 bore, but "squeeze bore" is just one of the common terms used to describe the feature. Additionally, I wasn't aware that $320 for a pristine Mini 30 with a Hakko scope and a bunch of accessories, magazines etc. was overpriced. The scope alone sells for more than that so in essence this is a "free" rifle.

Inaccuracy, as a possibility because I haven't even fired it yet, is the reason for why this thread exists. Gauging the possibilities beforehand is often a good idea and, out of sheer curiosity, I wanted to know what practical limits of lapping a barrel might be. Mission accomplished, thanks to everyone for their input.

 

[rcmodel]

"...it's a "squeeze bore" gun..." No, it isn't. There's no such thing.

Yes there is, or was.

http://weaponsman.com/?p=21443

rc

 

[Sunray]

"...leftover barrels from abandoned Ruger XGI project..." There were no "left over" barrels. Ruger never built any XGI's other than test models. Their marketing department prematurely announced they were going to build it at the SHOT show in January, then, in June of the same year, announced it wasn't going to be made. Ruger's engineers couldn't get it to actually work.
When the Mini-30 came out, 7.62 x 39 ammo came from Norma and nobody else. There were no suitable .311" bullets either. Suitable being 123 or 125 grains. .303 Brit weights were and are too heavy. Lots of 110 and 120ish grain .308" bullets though.

 

[dcloco]

Why not swage .311 bullets to .308? Could easily have a machinist make a die just for the process. (if you don't already posses this capability at home)

 

[hq]

"...leftover barrels from abandoned Ruger XGI project..." There were no "left over" barrels. Ruger never built any XGI's other than test models.

No kidding? Not taking into account how late XGI was scrapped, there were few economically viable ways to get out of contracts already way past letter of intent stage with subcontractors and making use of a boatload of .308 barrels during next few years. A friend of mine was Ruger importer to a number of European countries back in the 80's, a personal friend of Bill Ruger, and we talked a lot about what was going on back in the day. After the XGI disappointment I couldn't bring myself to buying a Mini 30 instead and that has haunted me ever since. Until now, that is.

With the exception of claim of Norma supposedly being the sole supplier of 7.62x39 ammo (funny, back in the early-mid 80's the majority of my 7.62x39 factory ammo was either Lapua or Sako FMJ:s or Sako soft points), I can't see how repeating pieces of common knowledge that's stated in one way or another across FAQ:s, articles and discussion forums online has anything to do with the subject at hand.

 

[hq]

Why not swage .311 bullets to .308? Could easily have a machinist make a die just for the process. (if you don't already posses this capability at home)

.308 bullets are a everywhere so there's no need to modify .311:s, but I'd like to avoid the reloading option for now. Like I've said, there's little rolling your own can't fix after some experimenting with different variables, but an ideal situation would be factory ammo for plinking and hunting. I have a decent cache of Barnaul, S&B and Sako ammo in bulk cases and I'd rather use them, providing that the gun itself will group acceptably with some/all of them.

 

[rcmodel]

My money is on the SAKO ammo, with the S&B running a close second place.

The Barnual tripped and fell down on the steel FMJ jacket bullet I betcha.

rc

 

[vtsteve]

I wonder running a few boxes of these through the barrel could open it up a bit (evenly)? Maybe start with the .308 kit and work up to the .312's?

http://www.midwayusa.com/product/491049/tubb-final-finish-bore-lapping-system-303-caliber

 

[rcmodel]

Noooo!

There is nothing wrong with it.

The guy hasn't even shot it yet, so he doesn't know.

I have an early Mini-14 in a folding stock that used to shoot under 2 MOA when I could still see open sights.

He needs to get his hands on it first.
Then see what it will do with available ammo before he messes up the barrel lapping or shooting grit coated bullets through it!!!

rc

 

[hq]

My money is on the SAKO ammo, with the S&B running a close second place.

The Barnual tripped and fell down on the steel FMJ jacket bullet I betcha.

My predictions are similar. Barnaul FMJ ja JSP, copper-plated steel jacket, cheapest russki stuff around. My main use for it is full auto, but in some instances it has been decently accurate. My biggest fear is that they won't do a tight bore any good in the long run.

Vsteve mentioned fire lapping. IME it can be detrimental to the throat with high pressure centerfire cartridges and its polishing effect is limited, even though it does produce a technically correct effect by being more abrasive closer to the chamber than it is at the muzzle. Rimfires are ridiculously easy to fire lap, heating up the wax on lead bullets just enough to make it sticky, rolling the tip in powdered white clay (kaolin) and firing 5-15 of them through the barrel. If there are any meaningful burrs or dimensional variations inside the barrel, lead slug is the way to go, even though it requires a lot of patience and due care, to not to damage the rifling, crown or throat in the process.

 

[james layman]

Think about making .311 bullets .308 by swaging, or buy proper bullets.

 

[lefteyedom]

Ship it to Ruger

Send the rifle to Ruger with a letter stating your personal safety concerns regarding the "apparently dangerous" undersized bore....
You might just get a new barrel on the house.... Or a letter saying it is safe to fire...
In ether case you won't have damaged the barrel

 

[redbullitt]

I lap all mine with bullets. Lots of jacketed and cast bullets.

Maybe investigate some cast bullets with the gun? Not sure if it would play nice with the gas system as I have never gotten to play with a mini 30. If workable, I almost guarantee you would be able to get something sized right and shooting well. AND cheaply.

FWIW i would prefer the 308 version of the x39 since I would not have to cast or buy another projectile size. Its not like I would be buying loaded ammo anyways. YMMV of course !


I like the sending it in idea above lol. That'd be my first stop!

 

[hq]

I lap all mine with bullets. Lots of jacketed and cast bullets.

Maybe investigate some cast bullets with the gun? Not sure if it would play nice with the gas system as I have never gotten to play with a mini 30. If workable, I almost guarantee you would be able to get something sized right and shooting well. AND cheaply.

FWIW i would prefer the 308 version of the x39 since I would not have to cast or buy another projectile size. Its not like I would be buying loaded ammo anyways. YMMV of course !


I like the sending it in idea above lol. That'd be my first stop!

Unfortunately sending it in is pretty much out of the question. A ton of paperwork for temporary export from EU, courier shipping with special handling fees as finnish postal service has a strict "no guns" policy, and everything in reverse when it's shipped back. It'll cost more than the rifle itself. Additionally, the local importer has shunned minis for several years so no help there either.

I'll slug-gauge the barrel tomorrow at multiple points and try to borrow a borescope. I've researched the subject a bit and it seems that the majority of accuracy issues in Minis are related to rough barrels (dull reamers and rifling tools), factory installation of pressed-on front sight which can deform or bend the barrel, and incorrectly aligned and torqued gas block that can misalign the gas piston. Mini30 barrels are thicker than early Mini14 pencil barrels, meaning that they're less susceptible to excessive flex during firing.

Lapping by hand with a lead slug is the cheapest (though labor-intensive) way to fix all issues associated with roughness, providing that the chamber is properly aligned with the bore and the muzzle is re-crowned (just to make sure) afterwards. Cheap barrels like these tend to benefit substantially from cryo treatment as well, but I'll have to look into it later if nothing else works.

I also might try and shoot the rifle at some point and see if it even has any significant accuracy issues in the first place...

 

[hq]

Update:

I couldn't get a borescope but slug gauging revealed some interesting details. The barrel is .310-.311 throughout and .308 near (2" or so) the muzzle. The tapering, if there's any intentionally, seems to be very long or alternatively the front sight compresses the barrel enough to reduce groove diameter by a couple of thous. There seems to be a slightly tighter spot near the gas block but otherwise it seems fairly uniform. So much for .308 diameter, we've probably fallen victims of a rumor as this is a 1987 manufactured early (189-0076XX) gun, yet it definitely has a .311 barrel; it only appears as .308 when the barrel is gauged throughout instead of pushing the slug back to the chamber, which may have confused even some experts.

I had planned a short range session for today, but it's been rainy and windy all day - 40mph windy - so there's no point in trying to shoot groups in this weather. Wind drift at 100yd would be close to 3½" not taking gusts into account.

I've hereby named this project "The Sow's Ear".

To be continued...

 

[hq]

OK. Now the baseline accuracy test is done. Sort of. Meaning that I grabbed the wrong ammo case and only realized it at the range - no Sako, no S&B, just Barnaul FMJ and 90's Czech, fully lacquered KOPP ammo that is so bad that I haven't even wanted to feed it to my FEG truck gun. Not to mention gusty wind at around 30mph.

Cold barrel, 3 shots, 2½" @110yd. Not as bad as I thought. Scope was way off, someone had sighted it in at at least twice the distance. Following 5 shots, 3½", POI shifted down just over an inch. Fifty more rounds and the results were pretty much the same. The majority of stringing was clearly horizontal, some of which can be attributed to the wind. Crappy ammo didn't help either, but the results weren't nearly as bad as I had feared.

I had just enough hard linotype to cast one lapping slug, so instead of regular lapping compounds I used Mother's mag and aluminum polish; fine but highly abrasive grit, my "secret weapon" for lots of polishing tasks. I would've liked to perform a full lapping job but it takes a minimum of five slugs. In any case, gauging the barrel was already much smoother. Night & day difference. The tight spot near the gas block is barely noticeable. Groove diameter is still at .310-.311, with a possible, slight bias to the latter and muzzle stayed at .308 even though I extracted the slug through it twice. The bore is already considerably shinier than it was before. I haven't borescoped it yet but it looks promising.

I'll try to find time for a quick trip to the range next week. Better ammo and calm weather should show at least some improvement. If not, it's time to get some more linotype and take a good look at bedding, gas block mounting and other variables.