Unusual Pressure signs

[Wicked Hunter]

I'm not actually sure it would be condidered a sign of pressure but for lack of a better term thats what im calling it. I was working with some 250gr. Barnes TMZ's this weekend and thought i'd try cranking them up a bit to see if i could get some better accuracy out of them. I started out at 38.0gr of Lil-Gun and worked my way up in .5gr. increments. Things were looking fine till i hit 39.5gr. I started to notice the extracter was begining to tweak the rim of the case. The primers were still nice and rounded and i had no other signs of excesive pressure. I went ahead to 40.0gr and decided i better quit as the extracter was really wrenching the rim of the case now, and my groups were only getting worse. So i guess the question is. Has anybody else had this same issue? The extractor yanked the rim of the case hard enough to made a slight buldge on the head of the case. I wont be reusing this brass nor will i be shooting that load again, it's just got me curious. Is this really a pressure sign or is it possible that the extractor isn't engaging far enough into the rim and that is why this is happening?

[BD1]

A Quickload estimate, (really a WAG without more info), puts the 39.5 grain load into the 50,000 psi range, well over the Saami max of 40,000 psi. However, the extractor bending the rims is not necessarily a sign of excessive absolute peak pressure, rather it is demonstrating a gas volume/timing issue which is opening the bolt while there is still enough gas pressure in the chamber to keep the case plastered tight against the chamber. You're in WC's territory here so proceed with caution, if you proceed at all. I think the road ahead would involve buffer modifications and possibly a change in gas port location. If accuracy is going south, I wouldn't see the point.
BD

[wildcatter]

BD is right here. You are hitting what I call the "Wall". What is happening is the bolt is opening when the case is still stuck in the chamber, as BD has just said. This is the function of the carbine length gas port location and therefore a timing issue. Some guns will open sooner than others, but in ever case it's not a chamber over pressure, as a matter of fact the pressures of Hornady's Factory ammo are overly tame and the timing doesn't allow us to exceed the tame factory ammo, by much, before you get to the wall you are at now, wherein you are seeing the effects of the bolt opening before the chamber pressures drop.

Your concerns about not using the cases now, is not warranted, other than not looking all that pretty, they should be in great condition to reload, the rims are a little tweaked, but the cases otherwise should be just fine, your call.

But alas, I have a cure, and that one of changing the timing of your weapon, on a custom basis. I've been doing them for some time now, on my weapons, and it really allows you to reach for the outer limits and speeds of 2500fps and much more are available in the 250gr bullets, after this conversion, all that's necessary is for you to do a safety work-up, paying attention to the real pressure signs.

The real trick, in the timing, for Hy-Preformance-Loads is to make the weapon still function with the tame factory fodder, no small feat here either. I'm about to do the first one, for a paying customer, and he is a member here. PM me if any of you are interested..

..t

[Wicked Hunter]

Thanks for the info guys. I should have added i do have the Tubs Carrier weight system with the heaviest weight installed in the gun. I'm also wating for a back-ordered Tubs flat wire spring. The only concern i had with the brass is that the head of the case is no longer flat. Will this cause issues when the case doesn't sit flat against the bolt face? If anything i'd be worried about the firing pin not making good contact with the primer or the round not allowing the bolt to make it into full battery. I may try flattening the head back out and see how it goes.

[Hoot]

Thanks for the info guys. I should have added i do have the Tubs Carrier weight system with the heaviest weight installed in the gun. I'm also wating for a back-ordered Tubs flat wire spring. The only concern i had with the brass is that the head of the case is no longer flat. Will this cause issues when the case doesn't sit flat against the bolt face? If anything i'd be worried about the firing pin not making good contact with the primer or the round not allowing the bolt to make it into full battery. I may try flattening the head back out and see how it goes.

You are correct in writing them off. The few times I ever tried to straighten a warped head, it did more harm than good. I've thrown away a few pieces that had other kind of damage and yeah, it's tough, considering what they cost, but it's the right thing to do. How many are you out?

Hoot

[Wicked Hunter]

Just 6 cases. I'm just gonna throw them out. I have another batch of 50 coming from Natchez soon anyways. Along with the 275gr XPB's and another 24 250gr TMZ's.