Lapping a .308 barrel to .311?

[hq]

I came across an opportunity to buy a genuinely mint Ruger Mini30. It has only a few dozens of rounds through it and has been gathering dust for well over a quarter of a century. I've been keeping an eye out for one for years and this one is so ridiculously cheap that I practically have to buy it.

The problem is that it's a "squeeze bore" gun, with a .308 diameter barrel. One of the notoriously inaccurate minis and the increased pressure from squeezing a standard .311 bullet through worries me. I reload but not 7.62x39 and virtually all factory ammo is loaded with .311 bullets, some hunting loads being fairly hot, not to mention tolerances in some surplus ammo.

I've hand lapped a number of barrels over the years, which by default increases the bore diameter by fractions of a thousands of an inch. Barrel change is out of the question with a gun like this, so does anyone know if lapping the barrel to an ever-so-slightly larger diameter, full .002-.003", is a viable option?

 

[stoky]

I think your best options are either use handloads with .308 bullets exclusively, or go with a new barrel. Give Accuracy Speaks or Clark's Custom Guns a call.

 

[hq]

Thanks for the suggestion, stoky, but rebarreling is out of the question, cost-wise. The only thing that I'm really interested in is some insight in hand lapping a barrel slightly more than just for normal accuracy-oriented polishing and deburring.

 

[stoky]

well, good luck with that

 

[243winxb]

Not a gunsmith, but i see no need to ruin the bore.

Ruger RUGER® MINI THIRTY RIFLES are chambered for the 7.62 x 39mm
cartridge, and can use either standard U.S. military or factory loaded sporting
7.62 x 39mm cartridges manufactured in accordance with U.S. industry practice.
Do not attempt to use any other cartridges in this rifle, even though “7.62mm”
may appear in their names (i.e., 7.62 x 51mm NATO [.308 Winchester]; 7.62
Tokarev; 7.62 x 54 Rimmed Russian, etc.).

Note that SAAMI lists bullet diameter at .311" Also note that cheap ammo is for the most part, not accurate. Good luck. Hope this helps. http://www.saami.org/PubResources/CC_Drawings/Rifle/7_62x39.pdf

 

[Coyote3855]

I'm not a gunsmith either and I have no idea what you achieved with your previous barrel lapping procedures. I am often wrong, but it seems to me that it would be extremely difficult to hand lap a bore to increase diameter by .003 while maintaining consistency from end to end. How will you avoid also increasing the diameter of the long throat referenced below?

If it were my gun, I'd shoot it with .311 and see if the accuracy is as bad you expect.

Here's some info from the Brownell's website:

Ruger Mini-30 bore specs
This rifle is chambered for the 7.62 x 39mm cartridge. This cartridge, traditionally, uses a projectile with a nominal diameter of .311 to .312 inch.

At the time the Mini-30 was introduced very few bullet makers where producing .311/.312 inch bullets for reloading in the light 125-130 grain weight required. Ruger initiated the use of barrels with a groove dimension of .308 inch and a long tapered throat. The throat allowed the use of ammunition with .311/.312 projectiles by gradually squeezing them to the .308 diameter. In addition, ammunition loaded with more commonly available .308 diameter bullets could also be used.

Commencing in 1992 Ruger initiated a change to using .311/.312 nominal groove diameter, 1-10 inch right hand twist barrels in all Mini-30's. It was likely well into 1993 before all rifles coming of the production line incorporated the .311/.312 barrels.

 

[hq]

So far my hand lapping experiences have ranged from nominal to fivefold improvement in accuracy, working on cut and finished barrels. The most I've ever increased bore in the process has been .0008", measured by lead shot and micrometer. There are considerations like avoiding the throat and muzzle areas during coarse compound stages, but all in all I'm confident enough to try this.

Providing that someone with reasonable hand lapping expertise gives this large bore increase a green light, or alternatively explains why it might not be a good idea even when performed with extreme care.

 

[mavracer]

Conventional wisdom would say that lapping the rifling enough to increase the bore size would have more detrimental effects than leaving it alone.
My first question would be is "is there a problem?"

 

[stoky]

My first question would be is "is there a problem?"

indeed
243winxb also has the right answer.
.308 or .311, IMO your chances of winning bench rest matches are slim and none.
Slim left town.

 

[hamp sandwich]

Going from .308 to .311 isn't necessarily lapping... Probably more like wearing out. Seems it would run a TREMENDOUS risk of wearing into an uneven bore.

 

[Malamute]

Few if any of the older Ruger mini-14s are known for great accuracy. It isnt just the bore diameter youre dealing with. By the way, have you shot it much with different ammo to see what it will actually do before diving in on a project that may be detrimental rather than helpful?

Lapping a rough bore is a different proposition that trying to take a thou and a half out of each side. You may well make it so it doesnt shoot even as well as it already does. It sounds to me that its more of a theoretical problem than a real one. Without shooting it with a variety of ammo, its really a gamble.

Have you shot some Lapua ammo in it yet? Id work on bedding, trigger, and gas block before messing with the bore if looking to improve accuracy.

I dont recall who it was, but somebody posted elsewhere that had actually worked at the factory in some capacity, and they always had trouble with accuracy in the older mini-14s. Sure, there will be some that say theirs shoots great, but the norm is generally so-so to poor by most standards.

I think trying to lap out that much bore is far more likely to be detrimental, especially in a gun never known for shooting all that great on average.

If its a great deal, buying, reselling it and buying a CZ 527 may be a better idea if accuracy is important, youd be starting light years ahead from a mini-14/30.

 

[hq]

Few if any of the older Ruger mini-14s are known for great accuracy. It isnt just the bore diameter youre dealing with. By the way, have you shot it much with different ammo to see what it will actually do before diving in on a project that may be detrimental rather than helpful?.

I haven't shot it at all. It's in genuinely mint condition, barely fired, and now I'm trying to decide between flipping it for a profit and keeping it as an all-round "cheap" deer gun.

I'd really hate to sell it, I've been wanting a Mini 30 since I first saw it introduced as a new model back in the 80's and they're very rare around here. It'd also be a perfect replacement for my very similar .44 Deerfield Carbine, roughly doubling its ~100yd effective range, providing I can get the potential and - knowing the model in question - very likely accuracy shortcomings solved economically. With some luck it'll shoot just fine and meet the 3MOA requirement with no modifications.

However, the whole question has a broader and less model-specific aspect, in whether lapping a bore a couple of thousands of an inch larger has any drawbacks or considerations, assuming that the whole lapping process is done by the book and with due precision. It's better to treat this as an academic question at this point, instead of concentrating too much in any single firearm. I'd love to hear an expert's opinion about it in general, or discuss it with someone who has first-hand experience in hand-lapping barrels. Preferably more than I have.

 

[natman]

Leave it alone. You won't be able to lap .003 from the bore and you certainly won't be able to do it evenly.

If you want a match grade gun, a Ruger Mini is the wrong place to start. Using 7.62x39 commercial ammo is the wrong place to start. Accept it for what it is rather than making it worse trying to make it better.

 

[BSA1]

Well I lapped the stainless steel barrel of my .270. It now consistently shoots 1/2" groups at 100 yards.
I can't honestly say it was worth the effort.

It is my understanding that the purpose of lapping a barrel is to remove any rough and tight spots along with any very small burrs left over from the rifling process. If you remove .003" how much rifling do that leave you?

Mini 14/30's aren't known for shooting small groups. How will.you know if it is the gun, the ammo or you?

 

[rcmodel]

1. I don't believe it is possible to enlarge a bore .003" by lapping, without ruining the rifling.

2. A .311" bullet fired in a .308" bore will be a .308 bullet after traveling one bullet length into the rifling.

3. It will have no effect on accuracy, and very little on pressure.

4. Don't even try lapping it .003" over.

rc

 

[Jim K]

^^^^

I don't know how you could lap .003" without removing the rifling, even with a lapping slug. Please, folks, don't confuse what the OP is asking about with normal lapping to smooth out a rough barrel. The only way to take that barrel out to .311-.312 would be by reboring and rerifling, which is costly and not practical. The new rifling would have to be the same twist as the original, plus the same or slightly greater groove width.

The only reasonable option would be a new barrel. Or pass on the rifle, which is what I would do.

Jim

 

[joneb]

Could this mini thirty be converted to a 30 Rem. AR ?

 

[rcmodel]

Has anyone ever seen .30 Rem AR ammo for sale, anywhere, ever???

That's just a real bad idea for a guy in Finland!!

rc

 

[hq]

1. I don't believe it is possible to enlarge a bore .003" by lapping, without ruining the rifling.

2. A .311" bullet fired in a .308" bore will be a .308 bullet after traveling one bullet length into the rifling.

3. It will have no effect on accuracy, and very little on pressure.

4. Don't even try lapping it .003" over.

Thanks. That just about settles is. Groove diameter seems to be .300 so the rifling is pretty solid, but I'll probably settle for a regular, easy polishing with a lead slug. Rifling can (and has to be) protected by roughening up the lapping slug by rolling it between two pieces of emery paper and wiping the smoother groove sections clean after applying compound.

Requirements for accuracy aren't that high, 5-6" grouping within the point blank range of 200yd or so and 95% of shots will be fired offhand or on shooting sticks, at much shorter distances. Maybe I'll just give it a try, shoot a few boxes of different ammo and see where it's at now. I really, really want to keep it no matter what and if nothing else works, rebarreling is always an option. Like I didn't have a pile of ongoing, unfinished projects already...

 

[raubvogel]

Weren't the American AK barrels rifled for 308 bullets for quite a while?

hq, you are in Finland so I think your choices are different than ours. I do not know the legal issues with reloading over there, but I would seriously consider that before trying to modify the barrel (besides polishing that is). Main reason is there seems to be a better selection of target-quality bullets in .308 than .311, at least here in the US.

 

[highlander 5]

I can tell you one thing it's going to be a lot of work lapping out .003 by hand. One of the jobs I had was working on a manual honing/lapping machine. Even with the right equipment removing that much material you might not have enough rifling to get decent accuracy. How do you plan to measure your progress,I had various air and electronic probes to check my progress and my tolerances were as much as .001 to .0003. That last measurement was for a component for parts of the fuel system for the f414 engine of the F16 and F15 fighters. Oh .0003 was straight and round.

 

[243winxb]

Corbin has said ,swaging down a lead core jacketed bullet in a die does very little harm. But sizing down more than .005" may hurt accuracy, because the lead core will become loose inside the jacket. This is because lead does not spring back as much as the copper alloy jacket after sizing. So, i see no reason to not shoot a .311" bullet in a .308" groove diameter barrel. The PSI will not increase that much, if any, with a lead core bullet.

 

[lemaymiami]

I've never owned a mini but have done a bit of reading up about them. Most that work to improve accuracy seem to go with mini-struts, an aftermarket barrel stabilizer... If you consider that route there was an interesting post recently about adjustable gas blocks that included a site that makes and sells both the adjustable gas block - and a variety of struts to consider.... Here's their site... http://www.mini14adjustablegasblock.com/#

They might also be a source of info on whether barrel lapping is a good idea or not.... Hope this helps.

 

[hq]

Availability of .311 bullets is pretty good so that won't be a problem. I'd just like to stick with factory ammo for now. The usual accurizing tricks will be used, of course; stabilizer bar, glass/pillar bedding and re-torquing gas block with feeler gauges and torque wrench. This is just one of these somewhat academic what-if -threads, because I'm going to lap the barrel in any case. Once I'm able to hit a CD at 200 yards with close to 100% consistency, everything's fine. Minute of deer vitals aspect at that distance is really all I'm interested in.

 

[Sunray]

"...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.
You will never lap 3 thou out of any barrel. Lapping is like polishing and uses pretty much the same products. Doesn't remove much, if any, material.
Your options are to change the barrel or reload. If you haven't bought the thing, don't.