Home Forums Reloading Big Game Rifles Blew my first primer today.

And speaking of the Hornady manual, they have a good write up of what happens when you jam jacketed bullets into the lands. You'll raise pressures higher than if they were seated a tad off but still not get the same velocity they show.

No.

Popping a primer is nothing that dramatic.

Something happened to me today which can add a bit to the discussion of book max powder charges and why chronographs can help keep you out of trouble.

I have a nice Pac-Nor barreled .243 AI with some 2300 rounds through it so far. The AI version of the .243 gains about 5% case capacity over the standard so one can roughly figure adding about 5% more powder. OR, going by the 4 to 1 rule (velocity increases at 1/4 the increase in case capacity), if the standard case can get around 3000 fps with a particular powder and an 85 grain bullet figure another 1.25% velocity or a whopping additional 38 fps for the AI. So around 3040 fps should be a good computed max in the AI.

This morning I happened to chronograph my long time standard load for this rifle of 42 grains of H4350 with the Sierra 85 BTHP. For the standard .243 Sierra shows a max charge of 42.2 grains IMR4350 with that bullet at 3000 fps. H4350 is generally a tad slower than IMR and you can generally add another grain or so over the IMR version. Anyway, a slower powder and increased case capacity - 42.0 grains of H4350 should be a perfectly safe load by all accounts. Note that the max velocity Sierra shows for the .243 with any powder and that bullet is 3200 fps.

Four rounds over the Oehler 35 averaged over 3300 fps, a good deal faster than when I first worked up this load. Something was definitely going on. I stopped shooting that rifle immediately and tonight I discovered the cause and will rectify it.

The lesson here is that had I just relied on this book max powder charge, even with a nice safe fudge factor thrown in, I would still be blithely wandering in possibly dangerous territory for each shot, not to mention the cumulative stressing of the action.

The other lesson is that even if you have a nice safe load worked up, it sure don't hurt to check it again every now and then.

Can i ask what the cause of the problem was?

Four rounds over the Oehler 35 averaged over 3300 fps, a good deal faster than when I first worked up this load. Something was definitely going on. I stopped shooting that rifle immediately and tonight I discovered the cause and will rectify it.

The necks had thickened to the point that about half of the fired cases wouldn't accept a bullet and the rest were a fairly tight fit.

AI'ing really does eliminate trimming, they have been fired 26 times and haven't lengthened more than .002-.004" over their life. I only trimmed them once at 2X to 2.035 and after 26 firings they are still all between 2.037-2.039", max allowable length is 2.045".

I neck size them with a Lee collet die and only bump the shoulders back with a Redding body die a measured .002" when needed. Since they weren't lengthening it never really occurred to me to check the neck thickness, but either by firing and/or the 4 times or so they've been FL sized it increased neck wall thickness by only about .001-.0015". I compared them to the neck walls on some brand new cases of the same brand and lot to determine that.

Apparently that little bit of thickening in this custom chamber was enough to do the trick. Now I suspect that after some 2300 rounds the throat is also getting pretty rough although I don't have a borescope to verify that, but any roughening would certainly contribute to more pressure.

I could neck ream the cases but after 26 firings I'm just going to give them an honorable retirement into the brass bucket at the range. Going to prep 100 new cases and fireform them over the next few weeks - this time I'll keep track of the neck thickness as well as length.

Anyway, good thing I ran them over the chronograph. Even at that excessive speed the bolt lift felt normal and there were none of the famous "visible pressure signs".

Chronographs are good to have.