Beginner trying to understand barrel fitting bennies

[ummm]

If any experts can correct me on this, I'd be grateful! Trying to see if I understand what's happening when a smith fits a barrel in a production-type gun, say a Glock 34.

So from the factory, I think a production-type gun has a drop-in barrel. Thinking that it fits the gun to within some tolerance -- let's just say it can rattle around 1/100th of an inch in either direction.

So, if the barrel is sitting "crooked"...if it's not pointing exactly straight, then that distance from dead-perfect-straight creates an angle, and I can Google the trig I've forgotten to calculate how far off the bullet would impact. If a 5" barrel is crooked by 1/100th of an inch, then at 25 yards the POI is off by like 1.7 inches.

So if I go have a gunsmith fit a barrel, is it correct to think he can reduce the maximum crookedness at which the barrel might sit? If he can reduce it to 1/1000th of an inch, then that bullet misses by like 1/3 rd of an inch instead.

Is that the benefit of a gunsmith fit barrel? Is that not how it works? Thanks for any edu-muh-cation

[superdude]

barrel fit is not a matter of whether it is crooked or not (though there is some concern here). it's a matter of consistency in lockup.

a barrel with slop in the fit won't be pointed at the same spot every time. the shots will go all over. the consequence is that group size is large. say a 10 inch group at 25 yards.

a barrel with a tight fit will be pointed at the same spot every time. the shots will go to the same place. the consequence is that group size is small. say a 1 inch group at 25 yards.

you're thinking in terms of point of impact relative to where the sights are pointing. but barrel fitting is concerned with how consistently it will be pointing at the same spot for each shot, regardless of where the sights are pointing. consistency = accuracy. inconsistency = inaccuracy.

most barrels are capable of extremely good accuracy - if you take them out of the gun and clamp them in a device that holds them still. when you put them in a gun they are now subject to how closely they fit all dimensions of the slide (front and rear) and how consistently they are locked into place when they are fired. it's the amount of slop they have in the gun that results in the loss of accuracy.

sights can be adjusted to change the point of impact. but the real task is to get the barrel fit "tight" so that it locks in the same place every time.

Edited May 1, 2012 by superdude

[ummm]

Excellent explanation, superdude -- many thanks!

[Koppi]

I was just shooting today. I Shooter had a Kimber SS in .45 he said he only had about 6k though it. Locked up, broke the barrel link as far as we could tell. So! Find, a good gunsmith. I use Paul Grundhauser in MN.

Good luck!