Let’s Talk .45 Acp And G.a.p. Ballistics

[Armando Q]

I read somewhere that 45 ACP speed is not dramatically increased by an increase in barrel length.

The logic was: because it is a low pressure round the powder does not burn completely. I am not sure I buy the logic and wonder if any hand loaders on the forum can give their two cents worth.

Also, does anyone know how .45 G.A.P. is affected by barrel length.

I read that some folks have shot 45 G.A.P. out of their revolver? How does it perform?

Thanks,

Armando

[jasond]

i'm no expert, but i do load some .45 and it uses small amounts of fast burning powder, and i never get much unburnt stuff. i think most .45 would be losing pressure in a really long barrel and not get more velocity because the powder is used up.

in a revo, they don't count the cylinder as barrel length so the barrel is really longer than in an auto, but there is that pesky leak partway down so you end up with the same performance roughly - a 5" revo should get the same performance as a 5" auto.

gap has less room for powder, with heavier bullets it's exaggerated. should make major easy though. hope this helps!

[357454]

I have chronographed several of my 625s with the same loads, data follows

gun/bbl Bullet bul wt Primer Powder Grns Velocity

625/4 West Coast RN230 Fed Bullseye 3.1 547

625/5 Mont. Gold 230 Fed Bullseye 3.1 563

625/6.5 West Coast RN230 Fed Bullseye 3.1 550

625/5 West Coast FP255 Fed HS700X 4.7 808

Kimber West Coast FP255 Fed HS700X 4.7 809

625/4 West Coast RN230 Fed E3 Select 2.7 600

625/4 West Coast RN230 Fed E3 Select 2.7 648

The last load listed was with a 45GAP with the bullet seated to 1.09 OAL

As you can see in a revolver the difference between guns makes as much difference as the barrel length.

OAL, in my gun at least seems to effect velocity as well.

[Armando Q]

I guess I should have done a search before posting. Apparently a bunch of folks have tried the gap and like it.

However, I am still curious about the whys of the ballistics

[Patrick Sweeney]

Pour some coffee, it's Didactic Learning Experience time.

A firearm and an airplane have some things in common. there are four forces at work on an aircraft: thrust, drag, lift and gravity. Inside a barrel, the equivalents for a bulelt are pressure, bullet mass and friction. Outside, the forces are momentum, drag, lift and gravity.

In almsot all cartridges, the powder is consumed (or not) before the bullet has traveled any significant distance. Even in something like a .30/.378 Weatherby the bullet cannot have moved very farf before the combusiton is complete. What's left is pressure equalization.

In most calibers, the force of engraving the bullet by the rifling, and the friction of passage down the bore is inconsequential. However, we can shift the variables so that it becomes more important. To do so we decrease chamber pressure. We use fast-burning powders so the pressure drops off as quickly as possible. We use a heavy, hard bullet. And we make it large in caliber and short in diameter so we have the most amount of bearing surface (and thus friciton) as possible.

Sound familiar?

I once had a 16" .45 ACP AR-15 carbine. (Wish I still had it.) With normal powders the velocity increase was small. Sometimes non-existant. After the (planned for the load) first five inches of barrel, pressure had dropped so much that friciton became significant, and thus ate away ant any potential velocity increase the longer barrel might have provided.

To avoid that, I could have loaded slower powders. slower powders have a slower pressure drop-off, and thus would maintain a higher balance against friction in the longer barrel. But it would have increased bolt thrust, and in a blowback that's not good.

In handguns, the frictional effects of small differences between bore diameters and surface roughness can make as much difference as an inch or so in barrel length. and a small difference in cylinder gap can be worth more. I have a 4" 625 that delivers more velocity than my 6.5" 25-2. Go figure.

[Viggen]

Sometimes the cylinder gap is a factor, sometimes not. Even if it should make a difference sometimes the numbers don't go that way. Sometimes it's a "fast" barrel that affects the most. Maybe.

This was with 45ACP not GAP

625 5" 3.7 gr Solo 1000 230RN(Molly Coated) 757fps

Colt GC 3.7 gr Solo 1000 230RN(Molly Coated) 739fps

I thought those results were interesting.

Edited August 29, 2006 by Viggen

[Armando Q]

Thanks for helping me understand!!! So, if I get this correctly…

While keeping all other variable consistent, slower powders have slower pressure drop off and they maximize the use of barrel length. However, recoil is increased. The trick is to find the fastest powder that utilizes the complete barrel length.

Thanks,

Armando

[jasond]

you got it, more or less. the slower powders listed will give higher velocity at the same pressure which means more recoil, and it takes more of the slower powder to make the same pressure and the weight of the powder itself adds to recoil. they have more muzzle blast too because the pressure is higher when the bullet leaves.

all else equal the gap will have more pressure than acp with the same amount of powder because it's in a more confined space, which means higher velocity. 357454's results would confirm that.