What will different barrels and barrel lengths do to power factor?

[1eyedfatman]

I'm only been loading a few months with a Dillon 650 and only in 9mm. Loading for IDPA, USPSA and other shooting sports. Main pistol now is a Glock 34 gen 4 with KKM barrel. Here is my load at around 135 PF.

- Bayou 147gr coated bullets

- Titegroup 3.0

- 1.145 OAL

- mixed range brass

In IDPA, I will now start shooting CCP (Glock 19 gen 4 factory barrel) and Bug (M&P Shield). PF's for ESP/SSP/CCP are the same at 125PF and for Bug its 95PF. I've only chrono'd the rounds through my G34. In theory, what would I expect to see shooting the same rounds through my G19 and Shield as far as velocity and PF? And what might I need to consider changing to adjust the loads for those pistols to stay close to their PF limits?

[Steve RA]

I'd chrono the load in all the other guns, see what the results are for speed and also check for accuracy and then go from there.

[kneelingatlas]

You can estimate a 5% drop in velocity per inch of barrel length.

Edited April 19, 2015 by kneelingatlas

[dee loo]

I normally shoot a G17 in SSP, my 147 plated RN/Berry's or Precision Delta FMJ (I get roughly the same velocity with both) load is usually 135 pf with 3.8 with WST, at 1.130, to maintain the same PF in a G19 I had to bump up to 4.0 grs. I had chronoed beginning of March in 83 degree weather. On the board I saw alot of post with 3.6 grs WST for the same pf in a G34.

Shot the G19 at last months Florida State IDPA match in CCP.

[presadad]

I'm loading 125gr bullets for an M&P Pro, a G34, and a G19. The same load will average 1050fps in the G34, 1035fps in the M&P, and 1000fps in the G19.

[Garrett]

I have found the individual barrel makes more difference than anything. Yes - a longer barrel will generally produce higher velocities. But that is not always the case.

I was just running a load through the chronograph yesterday. I was testing it in three different guns with different barrel lengths, and the results were interesting.

5.5" barrel = 1002.6 fps

8.375" barrel = 1083.0 fps

10.2" barrel = 1003.4 fps

The long and short barrels shot almost exactly the same average velocity (very similar high/low velocities as well), while the mid-length barrel shot significantly faster.

If I was using these in a match where power factor was measured, only one barrel pushed the 124 gr. bullet over 125PF.

If I had developed the load in the 5.5" barrel, and presumed it would be over 125PF in the 10.2" barrel, I would have been dissappointed.

As noted by others, you need to chrono the load in each gun you intend to use it in.

[SHARPPOINT]

Garrett Info.... +1

[kneelingatlas]

I suspect the burn rate of the powder also has an effect. Garret, were you using a relatively small charge of fast powder? Perhaps if the charge reaches peak pressure before the bullet leaves the barrel there isn't much to be gained with more length.

I have a few Open guns with 4 1/2" barrels which really take a hit in velocity when compared to 5 1/2" barrels, but I'm using large charges of slow powder and I suspect I'm not burning the entire charge before the bullet exits the barrel (queue large fireball!).

[Garrett]

I suspect the burn rate of the powder also has an effect. Garret, were you using a relatively small charge of fast powder? Perhaps if the charge reaches peak pressure before the bullet leaves the barrel there isn't much to be gained with more length.

Yes, but...

This was with a mid-range book load of Win 231 with a MG 124 gr. FMJ. I've found even the fastest powders are still burning at the end of your typical rifle-length barrel. True, the incremental increase gets a lot less, but you do still see some increase.

For example, I had a similar 231 load but using a plated 115 gr. bullet. In the same 5.5" barrel noted above, this load was averaging 1238 fps. In a 16" barrel, it still increased to 1410 fps.

I suspect how tight the chamber and barrel are can play as much into the issue as anything. In the example I gave above, the 10.2" barrel is a surplus machine gun barrel. So it probably has somewhat generous chamber tolenrances, where a target barrel would be tighter and likely give higher velocities because of it.

Edited June 2, 2015 by Garrett

[kneelingatlas]

All things being equal I'm still going to stick to my 5% per inch estimate, at least for my purposes: using slow powders in Open guns with barrel lengths between 4" and 6". This figure is based upon my results using barrels of the same make and rifling, so I think whatever role the barrel plays should be neutral. Now between CZ barrels, STI barrels and Schumann barrels I see huge variations