Shoulder bump question

[Wolfdog91]

So been playing around with this 17-5.56. now this is the first thing I've ever loaded where I have to do some case reforming. I made some brass up per the companies instructions and fire formed some today on my first tests.
Reformed case 1.450



Fire formed case : 1.458"



So if I'm understanding everything right like bumping my shoulders back on my initial reform like .005" more then is needed.
My question is will numbing the shoulders back just the .003 I need for my AR instead of .008 cause a problem as far as forming goes ? Or should I just wait till their fire formed to do the small bump ?
Sorry if it's a dumb question but first semi wildcat so not 100% . Super fun though really enjoying making my own brass up

 

[RegionRat]

The first "rule of thumb" is... it has to fit the chamber.
The middle rule is... if you over size the brass, don't expect a long life.
The last "rule" is... it has to cycle with reliability.

I don't run 17, but with 20P I treated the brass like any AR brass.

All you are trying to do is balance between sizing enough to run the gun smoothly, and not over working the brass. The shoulder bump only needs to be enough to fit the chamber and allow the gun to run. You don't need to over work that brass.

If you have the Go-Gage, I would use it to reference the chamber and then use that to set up your bump process. If you don't own the go-gage, you go by trial and error with a small number of brass samples to get more cycles sooner and not make big batches till you have established the bump.

 

[jepp2]

Are you changing the shoulder angle during the fireform?

I would adjust sizing until you find the case head to datum that gives you bolt contact. Then I would size back 0.002" from that. If the brass isn't fully formed to the chamber, just blindly setting the shoulder back 0.003" isn't a good idea.

 

[Coyotefurharvester]

First does "fire formed brass" chamber correctly BEFORE sizing(spent primer removed) ???
1. Yes, size without moving the shoulder numbers.
2. No, start with die set to size 1.456 ,does it chamber without forward assist?
A. No, move shoulder another 0.001-0.002
1a. Repeat until brass chambers
B. Yes, seat a spent primer 0.010 proud and chamber, remeasure (do several cases)
1b. If primer more than 0.003 proud after chambering, back die out slightly, if less than 0.002 to flush turn die down slightly..
This method should get you thru jepp2 questions and suggestions.
If it was a straight 17/223 you could just use a 223 ammo checker to size brass, but you have a wildcat.
If you are hunting, losing brass frequently, just need rounds that will function EVERY TIME and always using used brass to start with--the 0.002 headspace is meaningless. Lock your die to produce brass 1.453-1.454.

 

[NorCalMikie]

The first "rule of thumb" is... it has to fit the chamber.
The middle rule is... if you over size the brass, don't expect a long life.
The last "rule" is... it has to cycle with reliability.

BINGO!! If it was a bolt gun, you would be working differently.
You CAN bump the shoulders on a "gas gun" but you better know what you're doing.
I have bumped shoulders on my M1A just to say "I did it" but it was more trouble than it was worth.
The more you work that brass, the shorter it's life will be.
Every round fired will grow that brass. The closer it fits your chamber, the less you have to move that brass to fit again.