My first thoughts are the bullets are off balance. They might have voids in the cores that aren`t visible to the eye. Do they weigh close to the same? try checking to see if they have a large weigh variation.

Here's the setup...

Been a happy 40 S&W owner (Glock 22, BTW) for 8 years...well over 10000 rounds fired, over 5000 are reloads...

Today at the range, 5.7 Gr. Hodgdon Universal, 165 grain generic bulk JHP, Fed. 100 primers, first time to combine 165 JHP's with Univ. powder, but used both with other combinations previously without problems, 10 long paces to target...

First 100 or so rounds went out fine. Accuracy was decent.

Fired 30 more rounds...several holes looked wierd, but with all the target damage from the previous 100, I wasn't sure, so I started aiming at a fresh patch of the target...

15 rounds into the fresh area...clearly shows 6 holes where the bullets went through sideways. At least 3 more holes look oblong, like they were starting to destabilize! I later studied the early pattern, and found 3-5 clear keyholes, and clearly mostly good entries.

Sooooo....I have a load that progressively starts giving more and more keyhole shots as it heats up!?!?

I have used this same batch of bullets with VN340 with no problems. I have used this same batch of powder with 180 JHP's with no problems. I can't imagine it to be a twist rate/bullet stability issue.

I am at a loss. Any ideas as to what could cause this?

Ol Joe might be right, but the eccentricity would have to be severe to make a properly-spun bullet keyhole. I would also check the bore to see if it is picking up any serious fouling. I really doubt that your twist is too slow.

I cleaned it last night after posting the original article. I did think that bore roughness could be damaging the bullets...

It had some hard fouling near (1-1.5" in from) the muzzle (not the throat area!?!?), but not too bad. It wouldn't come up with 50 strokes of a bronze brush and 1 hour soaking with #9, but a small flat screwdriver scraped it off with only light pressure. It was much less than the height of the rifling...more a dark patch in an otherwise mirror smooth bore. It is all mirror smooth now.

I can't see it being a twist rate issue either...hence my puzzlement!

If fouling isn't the problem, you may have bullets that are a bit undersized, I'd take some measurements on a sample and see if they are on the small side. Only keyholes I"ve ever gotten were when bullets were too long and too slow or when they were undersized.

good luck,
graycg

Undersized would be my first guess too. Can you look at any of the keyholed bullets and check the rifling marks? Otherwise fouling would have to cover the rifling and that would have to be pretty severe I'd imagine, at least more than what I've seen.