Although I am not a regular contributor to this forum, I do regularly follow the entries. 

I have recently experienced a problem that I have not encountered before and which doesn't make too much sense to me.

I regularly shoot a Remington 40x in .223 at 100x. which has a 29inch Bartlein 5r barrel, 1 in 7 twist.

The range restrictions dictate that all callipers are restricted to a maximum muzzle energy.  In the case of the .223 with its small bullet this converts to a maximum muzzle velocity 1250 fps.

Although I shoot 308 at longer ranges, I have shot this rifle at 100x, under these restrictions, since I had it re-barrelled about a year ago, with no problems.

My reduced load for 100x with 59gn Sierra Match Kings is: 9gn of N320. giving excellent accuracy and around 2020fps.  The cases have always expanded normally, sealed the chamber and left the bullets a loose fit in the case neck, as normal, even with this low load.

Last week I fired the rifle at 600x with Berger 90gn bullets and 22gn of N140, vel. 2291fps.  I found that, with all the rounds,  that a new bullet would not enter the mouth of the fired case and evidence of powder residue down the side of the case and in the case extraction groove.

This completely mystifies me.  How is it that a light load performs as one would expect and a full load leaves the case clean and in a normally expanded condition and a full load shows evidence of not sealing the chamber and leaving the neck constricted?

Has anyone else experienced a similar problem and what do you think could be the cause?