Hopefully this is an appropriate place to ask my question, pretty new to the forums and this is the only ballistics related spot I could find.

In short I'm wanting to make a rifle chambered in .30carbine.  I've always liked the little round and it'd be a great stretch for my hobby level machining skills.  This main issue I'm facing is I cannot find anyone who sells a 1in20 twist .308 barrel which is the old military standard.

Plugging .30carbine M1 ball into the Miller stability calculator over at JBM balistics gives me a 5.7 stability factor with a 1in12 twist barrel.  I've read some articles warning about "overstabilization" and that if your stability factor is that high you'll have serious issues.  Others say that this is a myth and that high stability factors are fine.

My question is if anyone has experience shooting loads with very high stability factors like this, or any other expertise.  I've only read a few articles so I have a rudimentary understanding at best.  I'd just like to shoot this rifle out to 100yds or so with reasonable (i.e. a couple MOA) accuracy. Think 1in12 will cut it?  Will even a 1in17 (very pricey) work you think?

I don't see why it won't work, the problem normally with to fast of a twist rate is the bullets being tough enough to hold up and not disintegrate when fired.....

Run your numbers over
Bergers stability calculator..I think you will find a 12 twist is fine..

https://bergerbullets.com/twist-rate-calculator/

I will add this..I have shot 125 grain 308 cal barnes solids out of my 300 rum at almost 4000 fps..no issue at all with a 10 twist rate..Barnes even offers load data for their lightest 30 cals in my rum with a 10 twist...My rum now carries a 9 twist bartlein, I wouldn't hesitate to shoot the 125 solids thru it..

Haha, looking up a.30carbine BC, that was fun.  So got the same with that calculator you linked, 5.7 with a 1in12.  Appreciate the input from a real world perspective.  Should only be pushing these 110grs around 2000fps so I don't think theres much risk of my bullets flying apart.  Looks like revolver rifle will get to happen :).

Follow up though, it traditionally uses a round nose bullets but there are a lot of great spitzer .30cal bullets in lightweight options....

Huh, I looked up this load(over the counter 30 carbine) and used the listed BC they provided.. I get 1.77 stable with a 12 twist.....
https://www.cheaperthandirt.com/product/hornady-critical-defense-30-carbine-ftx-bullet-110-grain-2000-fps-25-round-box-090255810301.do?sortby=ourPicks&refType=&from=fn&ecList=7&ecCategory=201427
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I routinely shoot 110gr .30carb bullets out of my .308 at 3200fps from a 10 twist. I've even done it in a 200-500m match. They were great up to about 400m where the energy petered out and they stopped knocking the targets over reliably.

Over-stabilization is something of a misnomer. A bullet is either stable or unstable, there's no such thing as too stable. There are potential negative effects of excessive spin and some of it depends on your specific rifle. Spinning those pills too fast can actually cause them to come apart but that's based on several factors intersecting. I have not had them blow up in mid-air yet despite the huge velocity and tight twist and you certainly won't from a .30carb with velocity capped at not far north of 2000fps even from a bolt action rifle. The other major issue to be aware of is excessive spin aggravates any inconsistency in mass distribution which can open up groups. Over spinning can also lead to other issues but none that are likely to be relevant with a case that tiny.

Length is wrong on your calc. .30carb bullets are closer to .64" long. That will radically alter the value you got.

Yeah when I did the math was using .65 as bullet length.  Just measured some I had lying around as I couldn't easily find a number online for bullet length.

Appreciate the input!  Sounds like I need to do this and just see how it goes.  Certainly not planning to lob .30carbine out to 400 yards anytime soon

Ah, that makes sense..I guessed 1" because no length listed..knew they were pretty short, didn't realize they are that short...thanks...

Should be able to get 1:15 from Krieger or Bartlein, or other big custom manf.

Bartlein will go up to 1:18 on a 308 barrel...

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FYI, the ballistics spreadsheet I publish (which is free) has a huge list of bullet length/weight/bc data in it. Super handy to have on hand. My handle + dot com.

But I can get a .308 barrel blank from GM for like $50 which is pretty significant.

What kind of rifle are you putting this on? I'm assuming not an M1 carbine.

Lol here's where the judgement comes in... Thinking of converting a Ruger blackhawk to a rifle :D

Don't hate, revolver rifles are cool.

I also played with the idea of a bolt gun but I can't find a cheap modern one with a bolt face smaller than .30carbine.  Need to see about making a completely new bolt head from scratch to see if I can do a bolt gun.  A little ruger ranch or savage would be neat.

You can use a .223 rifle. The bolt face, magazine and extractor on a .223 should work fine. In fact, a rifle that accepts STANAG (AR-15) mags would be slick set up as a .30carbine. You could do loads set up with actually pointy bullets. Of course at that point you've basically built a .300BLK.

Please try to take this next bit as an honest concern for your safety based on extremely painful and relevant experience and I'm not capping on your plans. I love the idea of a revolverbine, just not the actual practice of one that runs such high pressures as a .30 carbine. It's flatly not safe. Hang on before you get mad while I tell you why. A black powder .45LC or even .45-70 for that matter doesn't make nearly the pressures (less than half) regardless of the total gas volume and so isn't nearly as dangerous. Cylinder gap flash from a high pressure round is not something to get your flesh near. .30carbine is pretty high pressure, just above .357mag pressures. I have a not little notch in one of my index fingers and some nerve damage from an occasion where my finger found its way near-ish to the cylinder gap flash of .357mag that was loaded pretty mildly (158's at 1100) to always remind me of that. It was a malfunction of the gun and a stupid instinctive response and also a long story I don't want to rehash ATM.

So to my point. I shoot a .357mag Dan Wesson with the cylinder gap set to .002" (extremely tight by revolver standards).Here's my .357; the one that bit me, in the hands of a skittish scottish lady student of mine with factory WW 110gr JHP's at the exact moment required to demonstrate why I'm so worried for your personal safety.

My questions is: "Where in this picture do you plan to put your non-firing hand when this beastie you want to build is configured as a rifle?" Feel free to discuss the question in any way you like.


As a matter of fact, I tell ya what... I've got a Mossberg MVP .223 and 10, 20 and 30 round mags and I own a m1 carbine and have a crap ton of both ammo and brass so I can check feed/fit/function on the MVP platform. Why don't I test the combo out for you? Maybe you don't end up with a nasty burned gash on your left wrist.

In no way would I get irritated at someone being concerned by my safety :D.  I've got experience shooting a little .357 as well and am used to a bite if my hand is near the cylinder, it can be quite nasty.  However it's only if I'm quite close  in my experience (2-3 inches). With a fore stock on this rifle and holding is as a lever gun I'd have seperation between the cylinder and my forearm.  This of course would need some testing to verify it's safe, probably first just near a piece of plywood to see how that fares.  Secondly I'm starting with a revolver chambered in this round, I'm merely adding a long barrel (and adding furniture and redoing the grip geometry) so I have no worries of the gun actually exploding.  Third, pressure and flash arent neccesarily correlated as I understand it, so although the .357 and .30 carbine have similar pressures they won't necessarily have similar flashes.

That being said .30carbine may be worse.  I think I'm at the point of trying it, and if it is dangerous (dead plywood) convert it back to just a pistol and enjoy it that way.  I'd be out the cost of the barrel which seems acceptable to me.

Now, onto your mossberg MVP.  I've had decent luck getting about 10-12 rounds of .30carbine into a stanag mag.  Don't care that it's not a full 30.  My principal concern is that the bolt face is too large for safe operation and reliable extraction of the case.  Looks like .223 is about .02 inches larger.  If you'd be so kind to check feed/fit/function I'd really appreciate it.

My skills on a lathe far outstrip my skills on a mill (and I have an el cheapo mill as well) so fabricating a brand new bolt is a bit beyond me right now.  If there was a bolt smaller than the .360 of the .30carbine opening it up would be retty simple.

Sorry that this is kind of becoming more about the build than ballistics.  Should it continue I'll move this over to the appropriate sub forum.

Actually went ahead and moved it over for further build discussion.

https://www.ar15.com/forums/armory/Unhealthy-obsession-with-30carbine/49-499530/?page=1

.30carb and .357 can really be thought of as damed nearly the same thing, especially in a single action revolver. The powder being used, pressures achieved and the breadth of the cylinder gap will be largely determinant of the size of flash that comes out but you'd be surprised. I think if you put a bit of double layer, properly thick leather between your meat and the gun's metal you'll be ok even with substantial flash. I'd think at that point the proximity to your face might be a little disconcerting but not dangerous at all. I've no worries of it blowing up. My only concerns in the whole matter are that I'm missing a small chunk of me from a gap of over 3 inches from my pointer to the cylinder and I couldn't stand by and let it happen to someone else without at least a jarring little note of "please use caution". Sounds like you're thinking it through smartly. Maybe take a piece of bologna and tack it to the plywood in the blast path. It'll give you a great idea of at what distance you can start expecting tatooing from gap flash. That'll be just a bit too darned close and you can refine from there. You could have a little metal shield made to redirect the blast upward on the left side. That would solve the problem outright. I'll try out the .30carb in my MVP for you tonight and let you know in the morning.

Ballistics wise, if you can get 125gr pills in the thing you can see a whole different world of trajectories and use cases.

Eat your heart out .300blk.

Great thought on the blast shield as well.

Had a small family medical emergency yesterday and haven't made it home yet. I'll get that test done for you tonight.