case stretch after firing

[jec]

I had a question that I hope you guys can help with. I was wandering what kind of case stretch you all had with a fired 6mmbr case that had been full length sized. My headspace is perfect at .001 and my reamer was made to my loaded rounds by Dave Kiff so I have a tight chamber. My full length sized brass measures 1.157 with my comparator and my fired unsized cases measure about 1.161 so they are stretching about .004. Is this about average?

 

[Preacher]

You really don't want them to stretch that much every time, something isn't right with your set up.
Pull the firing pin assembly on your bolt, and run a fired case into the chamber, it should allow the bolt to drop ALMOST to the bottom of the throw if it's headspaced correctly...
Put a sized case into the chamber and allow the bolt to drop, if it drops all the way to the bottom with out a hint of resistance your die is not adjusted correctly.
Tweak the die until you feel that little bit of resistance with a sized case in the chamber with the firing pin assembly out of the bolt......

 

[rayjay]

Put a .003' shim under the sizing die and then measure the cases. Since you have a method of measuring the shoulder bump you should adjust the sizing die to give .001 to .002' shoulder bump . It sounds like you are adjusting the die to bear against the shellholder like is taught in the mass market reloading books.

The die shims are available from the BR oriented vendors listed on this site.

 

[jec]

After I posted this message I thought to myself that I should have mentioned that this is a custom encore and not a bolt gun. The reason this should be mentioned is that with an encore you get some case growth even if your headspace is right because of the frame actually stretching when you fire the round. I was wandering what the normal case growth was in tight bolt gun. My encore has an oversized hinge pin to eliminate some stretching, and at this point the gun is shooting .24 MOA groups at 200yds during load development so accuracy is not a problem. Sorry I did'nt mention this was an encore in the first place. Thank you.

 

[NorCalMikie]

In your case you're probably oversizing the fired brass. You need to adjust your die so it 'just' touches the shoulder then try it in the chamber. Don't work that brass anymore than you have to.

 

[Travelor]

What dies are you using? Does the die have a collet or is it pulling the case over a neck expander after oversizing the neck by the non-adjustable die?

George

 

[7mmXbolt]

I think the questions should be how many times have these cases been reloaded and are you full length sizing the cases everytime to reload them? There is really no need to do this after the case has been fire formed to your chamber. Maybe after 5-6 reloadings you may want to bring them back in to spec but they shouldn't grow that much if at all. FL sizing everytime puts excessive stress on the brass and weakins the case wall. This will make them stretch more than normal. If this is the case, I would simply neck size after you FL size your brass the first time. That is typically done with new brass. There is one other thing that would cause them to stretch and that would be if you are changing your load make up from the fire formed case. A change in powder / powder charge / primer will result in different pressures. If the charge was increased for some reason or you went to different primer that would explain the excessive stretch your seeing.

 

[jec]

Thanks for all the input. Today I checked the bellm tc website, and Mike Bellm is an expert on thompson center firearms. I found an article on headspace and case length growth. From what I could tell .004 to even a mentioned .012 can be normal depending on what type of pressure and caliber you might be shooting. As I mentioned encores are sensitive to case growth and must almost always,except with a few of the small caliber rounds which exert low pressure)be full length sized between firings

 

[Mikem]

If that's the case, there's not much one could do, right?

On my 6BR, I use the Forster bushing bump die,not a full length sizing die) and have it set to bump the shoulder back to .001 less than the chamber length, if its needed. I then run them in a Redding body only die, but just enough to give it .001 case body clearance, if its needed. I want my cases to be as close to the size of the chamber as possible, and still chamber with out much bolt closing force. I dont want to be moving brass around much, every time. It shortens its life for one reason, and its accuracy for an other. But with that rifle, I dont see that happening.
M.