Questions about shoulder bump and body dies (after Erik Cortina's last video)

[TQB]

Hello,

After watching Erik Cortina's latest video I was left with some questions.

How many firings will it take to a new case to fully extend to chamber dimensions?

Instead of using a Full lenght sizing die (without shoulder bumping), during the first firings to allow the case to fully extend to chamber dimensions, can a a body die be used to keep the head of the case from growing to much?

Are those first firings, for the purpose of case extension, muzzle velocity dependent? Should hotter loads be chosen or will standard or mild loads enough?

Thank You,
Tiago

 

[Martin52]

Hello,

After watching Erik Cortina's latest video I was left with some questions.

How many firings will it take to a new case to fully extend to chamber dimensions?

Instead of using a Full lenght sizing die (without shoulder bumping), during the first firings to allow the case to fully extend to chamber dimensions, can a a body die be used to keep the head of the case from growing to much?

Are those first firings, for the purpose of case extension, muzzle velocity dependent? Should hotter loads be chosen or will standard or mild loads enough?

Thank You,
Tiago

The last time I ordered a custom FLS die I fired 3 x 0.308 Lapua palma cases 3 times, just neck sizing each time. The case headspace moved from 1.629 to 1.633. In my chamber that's about max before things get too tight, 1.6335 maybe but not 1.634.

Based on that I'd say 3 firings of that particular brass in my chamber will fully expand the case to chamber dimensions.

When all new cases have been fired this way I set my FLS to bump back to 1.632 and FLS each time after.

On your last question, the objective is for the cases to fully expand to chamber dimensions first to enable consistent set backs in future full length sizing. Regarding loads, I use a mid range load, and jam the projectile so no issues in primer ignition (because the new brass may be too short, happens occasionally eg h/s at 1.626/7 out of the box).

Ordinarily you would not need a body die unless shooting cases that have been fired out of a different rifle with a more generous chamber.

Martin

 

[Alex Wheeler]

Hello,

After watching Erik Cortina's latest video I was left with some questions.

How many firings will it take to a new case to fully extend to chamber dimensions?

Instead of using a Full lenght sizing die (without shoulder bumping), during the first firings to allow the case to fully extend to chamber dimensions, can a a body die be used to keep the head of the case from growing to much?

Are those first firings, for the purpose of case extension, muzzle velocity dependent? Should hotter loads be chosen or will standard or mild loads enough?

Thank You,
Tiago

It depends on the brass, some will fit the chamber after the first or second firing. Others like a 338 Lapua can take 4 or 5 for the base to grow. Just use your FL die, it wont stop the case from growing. Using a mild load for forming will help the brass last longer.

 

[RegionRat]

It does take more than one cycle to see a piece of brass finish forming to a chamber. The process is fairly straightforward for a bolt gun, but can be a little tricky with gas guns.

I agree with the others, in that you can still see changes out on the fourth or fifth cycle. The majority of the significant changes happen on the first two to three cycles for the majority of chambers. There are always exceptions. Old military chambers like the 303 Brit in the SMLE can have very large changes on the first cycle on the order of 30 to 50 thousands, but then slow to one or two thousandths per cycle on the second and third firing.

A body die can be used. It does mainly the same work on the case that a regular FL die would do, but doesn’t work the neck. The main point to the body die is for when you want to separate the prep of the body from the neck in two stages. As far as how you decide how much to bump, it doesn’t do anything different. As with any new chamber, you will always want to check how your sizing process is affecting the body and head, as well as the shoulder datum. As a rookie, you should measure more often and only back off as you gain experience.

Yes, there is a difference in using hot loads versus regular loads. This can affect the case head even more that it does the shoulder datum. Some folks like the first cycle on brass to be a medium pressure before increasing the charge. Many folks just use their regular charges from the start. If you are playing with very high pressure, you tend to have primer pocket problems much sooner and shorter overall brass life.

The experience comes over many cycles and many chambers. To start out, measure on each cycle and keep good notes. If you are running a bolt gun, it is easy to test your brass prep to know that the brass will cycle with an empty prepped case. This is a little tricky with a gas gun, but all the same principals apply.

With an AR platform, it is best to test a new chamber early on with just a few pieces with multiple cycles and establish your shoulder datum dimension before processing large batches. Otherwise, all the principals are the same. Reliable feeding on the AR platform takes just a little care.

 

[BoydAllen]

I suggest an experiment. Take some of your loading stuff to the range and repeatedly fire, measure and reload a single case. Set your die so that it does not touch the body of the case. The number of shots it takes to reach maximum shoulder to head dimension for a chamber depends on the brass, and how hot the load is. One trick that I have learned is to make a case that is tight from head to shoulder from once fired brass, by putting Scotch tape on the head. As long as you verify that you die is reducing the diameter of the body of the case, you can adjust the die down until you get the precise feel that you want, then measure and record that dimension. From that point you can use it as a reference when setting your die for the desired amount of bump, and to evaluate how near to tight in that dimension cases are. I did this for one of my PPC barrels and found that perfect sizing is .0015 over what a GO gauge measures.

 

[okie]

there is/was an expert on this forum on this very subject!!!....WHERE ARE YOU!!!!