I was checking the seating depth of 55 gr Sierra BK bullet in a custom 22-250 Ackley Improved and found that the depth decreased by .018, since last spring. I am wondering if there could be carbon build up in the throat area since the cases are considerably shorter from fireforming. I seat my bullets .010 off the lands and when I chambered a round I didn't notice any markings on the bullet which would now be .008 in the lands. Has anyone had this experience or did I err in my measurements? I am using a Stoney Point gauge with a case that I fireformed in my rifle. I took an average of ten readinigs which were all within a .001. I only shot less than 500 rounds in the rifle. Any suggestions. Thanks!

I guess it could be carbon. This is the first time I've ever heard of this. Carbon rings can be a problem but .018" is a lot of carbon. Were your original measurements taken with the same bullets? Were the first measurements taken before fireforming?

I'd be thinking your throat has eroded a bit in 500 rounds.

As long as it still shoots well, I wouldn't worry about it.

quote:

Originally posted by eddieharren:
I guess it could be carbon. This is the first time I've ever heard of this. Carbon rings can be a problem but .018" is a lot of carbon. Were your original measurements taken with the same bullets? Were the first measurements taken before fireforming?

I used a different bullet for fireforming, 50 Gr. Nosler BT, and a modified pre fireformed case for the Stony Point. For fireforming I seated them .005 into the lands. I then modified a fire formed case for the Stony Point using a 55 Gr. Sierra BK. They were not the same bullets but from the same lot. A friend of mine has a bore scope and I will have him take a look. Will you be able to see carbon with the bore scope?

I agree with Paul H

If your seating depth is decreasing then that means that your throat is eroding. I keep detailed records and in the calibers from 270 to 338 the throat erodes ~.005" every 20 rounds or so. It is not demonstrable every time and is not consistant but you can notice it as a definite trend.

I would imagine that in a hot caliber like the 22-250 AI it would be greater than in the calibers I am talking about.

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I am wondering if there could be carbon build up in the throat area since the cases are considerably shorter from fireforming.

The cases shorten from the brass moveing outward to full fill the chamber. There is nothing inthe throat that will shorten your cases.

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I seat my bullets .010 off the lands and when I chambered a round I didn't notice any markings on the bullet which would now be .008 in the lands.

What is your powder charge? It might be a compressed charge pushing the bullet out of the case over time.

I have found that different lots of the same bullet can vary that much. I now measure each lot of bullets I buy.

Jim

my ballistic tips shrunk 5 thou between boxs