Emmm... Do I Have A Problem?

[dakotasin]

went out today to shake out my new glock. i've never loaded for a glock before, so i must admit, i was a little timid about it. the gun ran like a champ w/ my sig's favored load, so no troubles there.

where i have questions is the velocity. the sig and xd-40 both have 4" barrels. i get a velocity of around 1175 - 1190 avg w/ those two guns running 150 noslers. today, i just clocked the same loads w/ the new glock, and my chrony spit out 1275 f/s avg.

ok, i realize the barrel is an inch longer, so i did expect to gain a little velocity, but i gained 100 f/s. the other two guns are very close to each other, and the glock is far faster.

so... does the polygonal rifleing of the glock tend to produce faster barrels? can 1" of barrel in a 40 s&w really account for 100 f/s? is my chrony just having a bad hair day?

of course, one of us lacked the foresight to go ahead and bring the sig or xd out, so i couldn't get a direct comparison... but, that load for the sig and xd was worked up over a year ago, and 100's and 100's of chronied shots have been consistent...

ideas?

[tpe187]

Generally speaking, you get about 50fps for every inch of difference in a barrel length. This is more true in rifles and is exponential in pistols depending on how fast a powder you using. Couple this with the ussual higher velocity from a glock's polygonal rifling and you are in the ball park. This is why it is important to chronograph loads through all guns. I actually did a similar test today. In a Sig P226 in 40 my 180gr load got an avg of 922fps, the same load in a Glock 22 got 932. Not a big difference, but add in different temps and time of day that you Chronographed, different barrel lengths and you could easily see a 100fps difference.

As long as you don't see any pressure signs and you arent working near maximum, I wouldn't worry about it.

[dakotasin]

thanks much - i was worried about my chrony being done in, actually.

no pressure signs, feeding, extraction, and ejection were perfect.

the load is 8.4 - 8.5 grains of power pistol (depending on what data you are actually using, this is a middle level load - speer #13 says 8.0 - 9.0, i have seen others top out around 8.8).