Thanks for all the suggestions! The barrel was installed and finished reamed by a local gunsmith, so for now we'll assume it was done correctly.

I'll try cleaning the bolt and resizing some of the formed rounds this week, and then start taking measurements.

Thanks!

Washougal Chris

The belt is only going to provide headspace is the chamber is headspaced correctly.
Did you put the barrel on yourself? Sounds like maybe the chamber is to deep. Are these barrels long-chambered, to have the threads turned to headspace?
Take some measurements off of your fireformed case(s) and compare to your die-formed. Not sure where the datum point is on the die, but maybe you can see if the shoulder on the fired case is significantly forward the die-formed.
If it is not headspaced, get it to a gunsmith pronto.
Then enjoy.

My A&B 416 Taylor barrel was short chambered.

I fired my 416 Taylor for the first time yesterday, and had some difficulties forming the cases. I took 20 cases of W-W 458 brass, and FL resized them in Hornady 416 Taylor dies, then loaded with CCI250's, RL-15, and Speer 350 gr bullets. Out of the 20 loads, 6 did not fire, although the primer was dented. I tried all mis-fires twice, and only one went BANG! the 2nd time through.

Could this be because the shoulder is not well-defined by the sizing die? Shouldn't the belt provide enough to headspace on until the shoulder is properly formed?

Rifle is a Mauser Mark X with an A&B barrel.

Thanks in advance!

Washougal Chris

Chris,
a "No Go" gauge will tell you if head space is too great, but you said the primers were dented. Could have the primers become contaminated with an oil, solvent, etc,? If the dent in the primer was light, and head space is OK, you might want to check firing pin protrusion and spring strength.