I've been having trouble with my 6.5 Gibbs, when I start into the top end loads with the standard powders (re22, IMR 7828, IMR 4831) I get high pressure with dismal velocity for a 29.5" barrel.
I've found that my fired cartridge neck diameter is the same as my loaded round neck diameter .288" ( my smith neglected to tell me it was a tight neck) how much should I turn the necks, is there a standard clearance? is this the likely cause of the high pressures?
H-1000 doesn't enlarge primer pockets like the faster powders do.
My rule of thumb is to make sure there is at least 0.0005" clearance between your bullet and neck when the round is fired. THat means 1/2 thou all around the bullet so your loaded neck diameters will be roughly 0.001" smaller then your fired case rounds.
A bit more comfortable is 3/4 thou all around the bullet and if your throat is quality you can even get by with 1 thou clearance.
I do not like anything much looser then this if your turning necks as it kind of defeats the purpose and often will result in your necks getting pretty thin and you have to then watch for neck splits more often and correct with annealing.
Simply put, if your loaded ammo neck diameter is 1 to 2 thou smaller then fired neck diameter, you are in the right ball park.
Keep in mind however that you need to measure this with relatively new cases. Best would be with freshly formed cases as the more a case is fired, the more it will heat temper and the result will be it will simply get spring. It will expand under pressure but then spring back smaller then chamber neck diameter so your measurements on that case will not give you a true idea of what your real fit it. Use fresh cases to get things set up and then your good to go once all is set up properly.
One other aspect to keep in mind is the base diameter of your chamber. If it is built to a standard 0.470" diameter, in most cases, this is quite a bit larger then modern 06 case head diameters are running and it will allow slightly more case head expansion then a properly fitted chamber. This is a common problem because for some reason most brass makers seem to think the 06 based rounds have a case head of a round 0.465" instead of the listed 0.470" correct diameter that most reamers are built to unless specified otherwise.
Try seating off the lands as well a bit. THis will also incease your pressure spike.
I would be suprised if RL-22 and H-4831 will not give you the best velocity to pressure ratio. Others to look at would be H-4350 and Rl-19. I would not use powders much slower then Rl-22 unless your using the real heavy bullets. For anything under 140 grains, Rl-22 will get you in most cases the most velocity performance. H-4831 will be nearly identical to that as well.
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I've found that my fired cartridge neck diameter is the same as my loaded round neck diameter .288" ( my smith neglected to tell me it was a tight neck) how much should I turn the necks, is there a standard clearance? is this the likely cause of the high pressures?JS
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Because the fired case is the same as the loaded neck, does NOT mean you have a tight neck chamber.
The necks on my 6mmBR are the same size as the loaded rounds, and I just punch the primers and reload them.
You might have a "No turn" chamber. If the necks are 0.288", and the chamber neck is 0.289" to 0.292", the fired cases will still be the same size, because they will spring back that few 2 or 3 thou.
The thing to do before cutting the necks is the measure the chamber neck - ask your smith. He may know the dia of his reamer, or he can cast it with cerosafe for you. DON'T just start turning cases blind, without knowing where you are going with it.
Just talked to the smith, after explainig the whole process, he gave me step by step instructions (which I didn't get to start with)
adjust die for crush fit (did that)
load a round with 54 gr H-4831 and a 140 gr bullet and fire
resize fired case and load with 62 gr of re-22 and a 140 gr and fire through chrony, then call him (to see if I survive perhaps?)
can't see how this will change things but I'll give it a whirl.
The destructions that came with my Romain built 6.5 Gibbs (way back when) said to run through a FL .270 sizer die, trim neck as far back as the length of the neck when it is formed (he enclosed one for a sample), turn the neck to .012" thickness.
Size through a 6.5 Gibbs die for crush fit (a neck turned and sized down piece of brass was included).
Load with 55gr H4831 140gr bullet (into the lands if necessary),then fireform.
I don't know about the neck thickness for your chamber but that worked out great for mine. I still have the two sample brass that were included with my dies and rifle.
I will post a pic of the sample brass later tonight.
Gibbs Brass Pics: Left is pre fireformed. Resized and neck turned. You can see the bump of the false shoulder. Post fireformed brass is on the right.
thanks for the info dave, don't know if I've posted this but I fireformed with 11 gr of unique/cow/tp and my necks are .015" thick.
Ray told me I was enlarging primer pockets from fireforming, this goes against all I know about reloading, the shoulders are still round.
He instructed me to fireform with 54 gr of H4831 then load up a charge of 62 gr of re22 and shoot it.
any load above 60 gr that I have tried has blown the primer pocket. what puzzles me is what is different about the different methods of making cases that would change this? the .003" of neck thickness?
the only powder I can run above 3150 with my cases is H1000, anything else and the pocket is gone with 1 shot.