Velocity Question

[Delta Dave]

The basic question: Why would different weapons give startlingly different velocities with similar loads?

Background: Spent some time at the gunshop last week talking about reloading and such. Ended up talking about favorite .45 ACP loads and the desire to make major power factor with the lowest charge of 231. I should point out that most of us load 200 gr. Precision coated bullets, either the SWC or RN. So to make power we need 825 fps average. This leads to my question.

One of the guys says he gets enough velocity with 5.2 gr. of 231. I on the other hand only get an average of 784 fps using 5.7 gr of 231. This has been verified with a chronograph, so barring equipment failure, I'm reasonably sure of my numbers coming form my weapon. In addition, the Speer #12 manual shows 5.6 gr. of 231 as giving 769 fps.

My friend opined that because he has a highly tuned 1911 with a fairly new, tight barrel that he is getting higher velocities than in my old, not so tight piece. This doesn't make a bunch of sense to me, can anyone shead some light on why the might be?

 

[Vern Humphrey]

Get a soda straw, and make up two spitballs -- one smaller than the bore of the straw and one a bit larger than bore size. Try blowing them. Which one takes more pressure to get moving?

Given identical loads, a slightly tighter barrel will produce more pressure, and hence more velocity.

 

[Nhsport]

I have a guess or two.
Overall length will change preasure and therefore velocity. The 45acp being short and stubby will show this more than other calibers.
Same primer? Different brands will sometimes give a fairly surpriseing change.
Use the same brand and lot case,same brand and lot of primer,same overall length,I assume you are useing the same batch of bullets the were sized the same.Your batch or lot of powder might be different and your scales might be throwing low or high.The test would be to use the same batch of ammo made by one goy and run it through the same chrono with two different guns.
The only different thing I can think about on the guns themselves is if they are different to the extreme so thet one is cycleing faster and opening up earlier so as to waste more gas preasure? Does the barell in the custom gun come from an aftermarket supplier that sizes the inside slightly different?

 

[The Bushmaster]

Bullet size, barrel wear, barrel roughness/smoothness (new to old), chamber fit, difference in brass from one piece to another even with the same brand. And numerous other causes to many to list. AND what Vern said...Hi Vern. Coming your way 10 July....

 

[huntershooter]

Personally, I would see your friends 5.2 gr. load chrono'd from his pistol. The above info is accurate and can be a factor. However, I'd want to verify your friends claim.

 

[dmftoy1]

Just to throw in my .02 (and probably worth less than that).

The "Gun Tests" gun reviews pretty routinely show two or three "similiar" pistols from different makers. They might have say 3 40 S&W's, with 4 inch barrels, one from SA, one from Kimber, and one from Glock. They shoot a variety of "factory" ammo through them and for the same batches of ammo they get surprisingly differing velocities. It's pretty consistent from one factory load to the next. (eg. the "fastest" pistol tends to be faster across all the factory loads though by differing amounts) FWIW.

Have a good one,
Dave

 

[Ol` Joe]

Tighter chamber
Tighter bore
Rougher bore
Longer barrel
Throat lenght and tightness
Leade lenght and angle
These gun related items can all add to the velocity of a cartridge.

Bullet hardness, jacket material
Core hardness
Bearing surfact lenght
Primer
Case capasity/Brand
C.O.L.
powder lot, or different lots of any of the above.
Canister grade powders have (I`ve been told) up to a 10% tolerance in burn rate. This is one of the reasons for starting low, and why the books don`t all agree on max charges.

 

[JDGray]

Using a Lee's manual on 45ACP loads, my 3.9gr Clays/ 230gr Rainier load, is almost identical to their data, at 730fps. My 231 load, on the other hand, was about 120fps slower than their data. Ya never know!

 

[Delta Dave]

Okay, I'm not smoking my running socks...

Thanks everyone. It seems that as contradictory as the claims might be, what my friend and I are experiencing isn't really that far out of the real of reality. His weapon is in fact considerably tighter than mine. His is fairly new, mine has some 30,000 or so rounds through it. I suppose I'm confused because my loads tend to track right along with the manual and his are quite a bit hotter. Oh, well, I learned something new, I like that.

 

[taliv]

i'll add to that list, the method of cleaning used.
some barrels can be a lot slicker than others, and fouling can make a difference