Load v. Bore Diameter v. velocity?

[Sparkitoff]

Interesting topic came up at a black powder hunt. A few Gents firmly believe the same load in a smaller bore develops more velocity than in a larger bore. They cite pressure as the reason. For example: 7/8-ounce measure of powder and shot in a 28-gauge v. same 7/8-ounce measure of powder and shot in 12 gauge. They say the 28 gauge will have much more velocity. I don't know. Does anyone have data - or (gasp) an opinion - or theory?

 

[BoomStick]

This calculator is a handy way to estimate pressure and velocity for different calibers, barrel length, etc.
https://www.p-max.uk/black_powder.htm

 

[Sparkitoff]

Thanks Boomstick. That is pretty interesting. The only thing slightly different is that it calls for bullet diameter, which I entered as bore diameter (since it's shot, but takes up the whole diameter). This says the 12 gauge 7/8 ounce load is 11000 PSI and 1307 fps while the 28 gauge with the same load is 16000 PSI and 1330 fps (with the powder I chose). I don't think the velocity is that high frankly, but the comparison is interesting. A few FPS less in larger bore but with 30% less pressure.

 

[Historian]

Nope. The opposite is true. You need to understand that "swept volume" comes into effect. Look at the most recent Bevel Brothers article in Muzzle Blasts. A 28ga is nowhere near as efficient as a 20ga or 12ga or 10ga for that matter. If all variables are the same, black powder granulation and grains, wads, and shot; barrel length is the key factor here as a shorter 12 or 10 ga barrel will produce higher velocity while it will take a longer 20 or 28ga barrel to produce the same velocity with more pressure. Key reason I shoot 3F in my 20's is because of this. This applies to smooth bores. Also, pressure is not a great way to analyze velocity as it is again only one of the key variables at work.

 

[Sparkitoff]

12 or 10 ga barrel will produce higher velocity

The conversation assumed all things being equal. Same amount of same powder, same type/thickness of cards, same amount of same shot...etc,. with same barrel length. If just the gauge were different - some of these guys think the smaller gauge has higher velocity due to pressure. I know barrel length adds another factor.

 

[Historian]

What they think or believe does not matter. Go by facts and facts alone. I have also noticed the "swept volume" principle in multiple shotguns. How you might ask? My chronograph tells me via the velocity output. I used to think the same thing too until I was enlightened and saw the chronograph results. Then I started to ask questions and found out about the swept volume principle. Best of luck to you.

 

[Iceman]

I wasn’t going to reply because I cant find my source to verify but you did say opinions or theory so I’ll go from memory the best I can. Historian is correct, and it does have to do with the volume of the barrel. To add to that as he noted to achieve the velocity of the 12 gauge you would have to lengthen the barrel of the smaller gauge. That is because the volume of the small cylinder (barrel) is getting larger by becoming longer and closer to the 12 gauges volume. The cylinder with the larger volume is more efficient. Aspect ration also favors the 12 gauge. If two cylinders have equal volume the cylinder with a larger bore vs length or aspect ratio is more efficient.

So 12 gauge will always (in theory) run faster at a lower pressure than the 28 gauge with the exact same load due to its cylinder being more efficient.

 

[TCMonts]

JMO, comparing the same load in two different gages of shot guns is not apples to apples. I believe that a similar stack will give similar velocities. Given the same powder, the same wads and the same shot you will get the same velocity from the same length barrels. Say for instance you have 1" of powder, equal thickness wads, 1" of shot and equal over shot wads and similar barrel lengths the velocities should be the same. a larger gage can give you a denser pattern or a larger pattern at an equal distance. I only judge efficiency by how well it kills game for me. I have known guys that are deadly shots with tight shooting small bores, but I'm not one of them. Just my thoughts and I'm not real smart so their worth exactly what you paid for them.

 

[tenngun]

just looked up some comparisons, they may not apply since it’s shot or maxi being compared here
A .45 maxi is 230 grains, in a 36 inch barrel 70 grains 3f gives a velocity of 1632 fps
A .54 round ball in a 34 inch barrel weighs 220 comes out at 1439
A .58 minie ball at 530 grains on top of 100 grains 2f in a 32” barrel gets 1046 fps a .75 shooting a 545 grain 715 ball gets 1006 fps
A 20 bore on a 3 dram load… heavy in a 20 bore, shooting a one ounce shot load gets between 1094 and1204 fps depending on wad
In a 12 bore one ounce on 3 drams gets 998- 1154 depending on wad.

 

[TDM]

Interesting information from all.