Barrel Length and caliber

[mcb]

This is totally a half baked thesis for a thread (social distancing induced cabin-fever is effecting my brain I think) and will likely crash and burn for not being thought out. Shoot it down if you want...

Why do we discuss barrel length without taking caliber into account? The big boys do. You rarely see the barrel length of a 105mm or 120mm or 5-inch gun given in a length (meters or feet etc) Pretty much all of the big-guns barrel lengths are given in calibers. The length is reports as a number that is how many times will the diameter of the projectile divides into the length of the barrel. ie the American Abrams use a version of the Rheinmetall Rh-120 with a 44 caliber barrel length while the Leopard 2 uses a 55 caliber barrel length of the same gun. This way of designation barrel length goes back to the time of the early breach loading black powder guns IIRC.

I think from a reloader's point of view it makes more sense to think of barrels like this. If you did it routinely I think selecting burn rates of powders and evaluating potential velocity losts and gains due to changes in barrel length would be come a bit more intuitive. A 20-inch 223 barrel is ~89 calibers long but a 20-inch 308 barrel is only ~65 calibers long and a 20-inch 45o BM barrel is only ~44 calibers long.

I need to spend some time in Quickloads and see if this correlates with anything... Anyone else think like this or see any good articles/research along this line of thinking?

 

[jmr40]

I think it is a lot more complicated than that. A 30 carbine and a 300 Weatherby magnum are both 30 caliber. But one burns 15 gr of powder, the other 90 gr.

 

[mcb]

I think it is a lot more complicated than that. A 30 carbine and a 300 Weatherby magnum are both 30 caliber. But one burns 15 gr of powder, the other 90 gr.

See, half baked... But I still think caliber length barrel has merit. But as you point out we also need to take cartridge capacity into account.

 

[Slamfire]

It is worth skimming through these to get an idea of how complicated a question you ask.

AMCP: 706-150 Interior Ballistics of Guns

https://apps.dtic.mil/sti/pdfs/AD0462060.pdf

AMCP 706-247 Ammunition Series, Section 4, Design for projection

http://everyspec.com/ARMY/ARMY-General/AMCP_706-247_JUL1964_55541/

Changing the size, configuration, was extensively studied for cannon, and it turns out, there are real limitations. You can't keep the pressures up for long duration burns, so the expansion of gases essentially stops. These same principles apply to small arms.

 

[GBExpat]

Calibre.

 

[buck460XVR]

I think it is a lot more complicated than that. A 30 carbine and a 300 Weatherby magnum are both 30 caliber. But one burns 15 gr of powder, the other 90 gr.

You are correct sir, gets even more complicated when you put a .308 Winchester in the picture.

The whole way small arms calibers are named, is confusing enough. Seems sticking to simple barrel measurements just makes life easier and more standardized. I mean, I can buy both .357 and .44 mag handguns with 6" barrels. Without the use of a hand held calculator, I know they are the same length. Do I use .44 or .429 for determining barrel length? Aren't .44 mag rifle bores different than revolver?

 

[mcb]

It is worth skimming through these to get an idea of how complicated a question you ask.

AMCP: 706-150 Interior Ballistics of Guns

https://apps.dtic.mil/sti/pdfs/AD0462060.pdf

AMCP 706-247 Ammunition Series, Section 4, Design for projection

http://everyspec.com/ARMY/ARMY-General/AMCP_706-247_JUL1964_55541/

Changing the size, configuration, was extensively studied for cannon, and it turns out, there are real limitations. You can't keep the pressures up for long duration burns, so the expansion of gases essentially stops. These same principles apply to small arms.

I am not asking a complicated question per say. It just seems that the relationship of bore to barrel length might be better represent in the way the big gun do it compared to small arms methods.

Yes I understand how complicated interior ballistics are I have done a fair bit of research in the area both personally and professionally. I have met and worked, briefly, with one of the authors of: Ballistics: Theory and Design of Guns and Ammunition (https://www.amazon.com/Ballistics-Theory-Design-Ammunition-Third/dp/113805531X/ref=sr_1_1?dchild=1&qid=1586359858&refinements=p_27onald+E.+Carlucci&s=books&sr=1-1&text=Donald+E.+Carlucci) in a job two jobs ago.

When you think about changing barrel lengths a 2-inch chance in barrel length is significantly more for a 22 caliber than a 45 caliber gun.

Yeah I am still half baked.... But I think it would have merit...

 

[Walkalong]

Sure, such as .357 Mag benefits from a longer barrel more than a .45 ACP.

 

[mcb]

Sure, such as .357 Mag benefits from a longer barrel more than a .45 ACP.

Right! No doubt case volume is playing an important role but your very much on point with my though process. Possibly an even better example is comparing 357 Mag and 44 Mag both are the same max pressure, the same OAL and being straight wall cartridge so case capacity is proportional to bore area. Now think how much greater the performance increase 357 Mag realizes going from ~6-inch to 16-inch barrel compared to 44 mag over the same change. 357 Mag ammo frequently gains 400 fps, even 500 fps in some cases but such a change in 44 mag typically only see a 200-350 fps gain. That same 10 inches of barrel length is 28-calibers for 357 Mag but only 23-calibers for 44 mag. 357 mag realizes 20% more barrel length when measured in caliber rather than actual length.

half baked rambling...

 

[Deus Machina]

This is going to change by the capacity and load, too. And function of a gun based around the intended powder.
Artillery doesn't account for that because it was already accounted for and pretty well standardized. Easy to do when it's for the military only and they can enforce that, not easy when you make a million guns for a million shooters across a hundred platforms.
For instance, a friend's dad found a load he settled on for his .357 lever gun. It would shoot out of the 4" S&W I had, but performance was terrible. Sure put on a heck of a show, though!

You would have to standardize on a middle-of-the-road, hopefully case-filling powder to get usable data at all. Obviously a fast-burning powder is going to max out its pressure quicker, and perform better (relatively) from a short barrel than a slow-burning powder that can keep pushing down extra length.

 

[redneck2]

I think short, fat cases use shorter barrels than, say rifle calibers that have a large powder capacity relative to bore size.

A .45 acp is short, fat, and the bore is relatively close to case length. A magnum rifle needs much more length than a .45 acp rifle

I suspect it has to do with powder capacity and the relative heat generation. After all, what really propels a bullet? Expansion due to heat. More powder=more heat=more expansion=more barrel time before the “push” quits.

Powder capacity compared to barrel diameter is the main factor in barrel life. More capacity = more heat = more muzzle blast and lower barrel life

A .22 rimfire uses a few grains of powder and barrel life is almost infinite. Move that up to a .22 Hornet with 11 grains of powder and the barrel life is 5-10,000 rounds. Make a .223 out if it and use 24 grains of powder and life is cut to maybe 5,000 at the most. Move up to a .22-250 with 38 grains and the life cuts to maybe 1,500 rounds

 

[TonyAngel]

Oh, I take this stuff into consideration all of the time. I mean, I don't apply rocket science or anything, but....I try to take not only velocity, but also acceleration into consideration. This may also be half baked, but...for example, let's take 9mm. If I'm loading for a full size pistol and want to try to duplicate factory load velocity, I'll use accurate No. 7 for longer barreled pistols. If I'm doing the same for a compact or ultra compact, I'll try No. 5, on the theory that the faster powder will develop more of it's velocity going down the short barrel than the slower powder will. Sometimes it works out. Sometimes it doesn't, but I can't be held responsible for what goes through my head while I'm doing this stuff.

 

[buck460XVR]

Right! No doubt case volume is playing an important role but your very much on point with my though process. Possibly an even better example is comparing 357 Mag and 44 Mag both are the same max pressure, the same OAL and being straight wall cartridge so case capacity is proportional to bore area. Now think how much greater the performance increase 357 Mag realizes going from ~6-inch to 16-inch barrel compared to 44 mag over the same change. 357 Mag ammo frequently gains 400 fps, even 500 fps in some cases but such a change in 44 mag typically only see a 200-350 fps gain. That same 10 inches of barrel length is 28-calibers for 357 Mag but only 23-calibers for 44 mag. 357 mag realizes 20% more barrel length when measured in caliber rather than actual length.

half baked rambling...

But....are you using the same weight projectiles for .357 and .44mag? Would you realize such a big gain if you use using 240 grainers in the .357 or 158 grainers in the .44?

 

[mcb]

I agree with you guys the case volume, propellant choices, and bullet weight etc etc etc cannot be ignored. That was not my goal with this notion of measuring barrel length in calibers, it was simply to put more emphasis on caliber when discussion barrel length.

I think the 308 family of cartridge is another good example. 243 Win, 260 Rem, 7mm-08, 308 Win, 338 Fed all have similar case volume but 243 is going to react more to the same barrel length change than 338 Fed will. 4-inches of barrel length is many more calibers of 243 compared to 338 and thus makes a bigger difference to 243 than 338 in many cases.

But....are you using the same weight projectiles for .357 and .44mag? Would you realize such a big gain if you use using 240 grainers in the .357 or 158 grainers in the .44?

So going down a rabbit hole I would use a scaling system based on the assumption density does not change would be fairest. This is fairly accurate since all bullets are made from very similar construction of lead and copper. So for a fair comparison we scale our bullet mass by a characteristic length cubed. In this case our characteristic length is probably best selected as caliber. So a 158 gr .357 would, scaled to .429, be roughly 274 gr.

 

[redneck2]

I think if you figured out a ratio of powder charge to bore diameter you’d be closer to what you’re looking for

I had a .35 WSSM, formed from .25 WSSM cases. The optimum length was 21 1/2”. I strongly suspect that the optimum length at .25 caliber is quite a bit longer

 

[mcb]

I think if you figured out a ratio of powder charge to bore diameter you’d be closer to what you’re looking for

I had a .35 WSSM, formed from .25 WSSM cases. The optimum length was 21 1/2”. I strongly suspect that the optimum length at .25 caliber is quite a bit longer

You're probably right since I am not sure exactly what I was after with this half baked idea. It just seem the aspect ratio (length to bore diameter) of the barrel is more important/meaningful than the absolute length but beyond that I have not come up with a solid thesis. Like I said maybe a little number crunching in Quickloads might clarify or burn-down this half baked idea.

 

[redneck2]

Interesting read. I think expansion ratio is your answer

https://en.m.wikipedia.org/wiki/Physics_of_firearms

 

[mcb]

Interesting read. I think expansion ratio is your answer

https://en.m.wikipedia.org/wiki/Physics_of_firearms

I think expansion ratio is a very good description of referencing barrel length in calibers instead of absolute measurements.

I took a first stab at it last nigh with my 357 vs 44 mag idea from further up thread. In Quickloads I created a 357 158gr and 44 274gr loads both pushed by H110 and both pushed to a max pressure of 35,000 psi. Then plotted barrel length and velocity both in inches and then in calibers. That did not do as much as hoped... But then I took the differential so basically the acceleration as each point down the barrel. Then plotted this barrel length vs acceleration in both inches and calibers and the difference between the two barrel length system was more noticiable and the calibers vs acceleration was interestingly close to each other. Far from exact, not that I expected it but it was interesting results. I will try to clean it up and make it presentable. I would like to do similar with the 308 family and see what happens their too but that is going to be a time consuming mess. But hey we got time right....