Measuring Gas Gun Shoulder Bump?

[RabbitSlayer]

I have a WOA 204 upper. I have fired about 300 factory loaded Fiocchi. Im about to reload the once fired brass. I measured some random unfired factory loads and the once fired brass with the hornady tool. The once fired brass is all over the place?

I think I once read that the gas may need to be disconnected to get it to fire form consistently to get the measurement? Is that correct? Would it be worth the effort? Or is the data below enough to work with? I'm going for .003 bump so with that in mind i thought 1.553 or 1.552 might be a good starting point?

I guess my most important question is if there is not enough bump can it cause OOB kaboom in an AR? Or will it just not fire?

In contrast i have a 6 creed bolt gun and once fired hornady match is very consistent. 10 pieces were all +\- .0005. heck thats probably beyond the accuracy of me and the measuring tools.



Thanks in advance!

 

[Sandstorm]

In my experience a Gas gun is going to bee all over the place...but more so the factory once fired.
Just bump em back enough to chamber easy and go from there.
You shoud'nt have any problems..and they are real hard on ejecting the brass as well so there's that.
Your numbers dont look that bad anyway...just load and shoot.

 

[jelenko]

Over 10's of thousands of rounds through an AR, my experience is the same as you've found - very little consistency in 'headspace' of fired brass.

Disconnecting the gas system ought to work.

These days I use one of two approaches.
1) Resize the brass so the max 'headspace' of any case is 0 on a case headspace gauge. For the Hornady tool, you could use a Go gauge as the max.
2) Adjust the resizing die so it just engages a first case when the press handle is all the way down. In smidgen increments, screw the die in, resize, see if the size case easily chambers. Once it easily chambers, screw the sizing die in another smidgen or two.
You can achieve the same result if you first put one layer of scotch transparent tape on the bottom of the fired case but stop when the case, with the tape on the bottom, easily chambers.

 

[jepp2]

I guess my most important question is if there is not enough bump can it cause OOB kaboom in an AR? Or will it just not fire?

Why not measure the case head to datum length that you have bolt contact when closing the bolt with the tip of your finger and record that? Bump back what you feel comfortable with from that point.

When you shoot brass that is too long case head to datum, either the chambering action will set the shoulder back slightly and the bolt will close, or the bolt will fail to fully close. When that happens you learn how to "mortar" the rifle to extract the bolt. If you have a collapsible butt stock, fully collapse it before you mortar.

The factory Fiocchi in 204 Ruger I have shot have been WAY short from the factory. Like 0.010" at least and they didn't even come close to fully expanding on the first firing. Generally primers were sticking out considerable after firing. I don't trust any measurement on once fired Fiocchi as meaningful.

 

[Coyotefurharvester]

If the fired brass all will go into battery(chamber without FA) what measurement will you base your 0.003 shoulder bump off of? If the 1.5560 will chamber properly without sizing, use a spent primer partially seated(0.010 proud) and drop the bolt on it in the chamber from 1/2 travel. Measure length change(how much primer still proud). If 0.001 or 0.002 set die(no expander) to move shoulder from the proud primer measurement. Or use a ammo checker to set the die.

 

[RabbitSlayer]

Why not measure the case head to datum length that you have bolt contact when closing the bolt with the tip of your finger and record that? Bump back what you feel comfortable with from that point.

When you shoot brass that is too long case head to datum, either the chambering action will set the shoulder back slightly and the bolt will close, or the bolt will fail to fully close. When that happens you learn how to "mortar" the rifle to extract the bolt. If you have a collapsible butt stock, fully collapse it before you mortar.

The factory Fiocchi in 204 Ruger I have shot have been WAY short from the factory. Like 0.010" at least and they didn't even come close to fully expanding on the first firing. Generally primers were sticking out considerable after firing. I don't trust any measurement on once fired Fiocchi as meaningful.

Interesting. I read that somewhere else too. I sort of thought it might need another firing to fully expand.

 

[jepp2]

The method described by @Coyotefurharvester will get you very close.

 

[Bronsin]

If I were to load for a gas gun, I would use a GO headspacing gage and some shims to find out what my chamber measured, then set my dies to size .003 shorter.

 

[Scott-ak]

I'm curious what you are measuring with? Many folks use calipers expecting precision measurement. They are good tools but even the best Mitutoyo are +/- .001 So my point is, if you are using calipers do not get hung up on those numbers and completely ignore the 10 thou position
This is what I've done w/o any better instruction, resize a case w/o bumping the shoulder. Insert the case into the upper. Hold it however you want but you want to push the back of the bolt by hand. It should not go into battery. Start bumping the shoulder back until the round and bolt will go into battery with light force as compared to doing it with no case. Keep in mind that you are overcoming the ejector spring so there will be some resistance when compared to no case. Bump .003-.005 past that point. This 'advice' is worth exactly what you paid for it

 

[dellet]

Something to keep in mind, so you don’t think you’re losing it.
If you are trying for minimum bump or set back at the shoulder, what is likely to happen is that you will back the die off to not touch the shoulder, then size a piece of brass and remeasure.

Your 1.556” piece may now measure 1.557” or more.

When you reduce the diameter of the case, the shoulder will move forward until stopped by the die, same as squeezing a tube of toothpaste. It can be confusing when you expect to be reducing the measurement and it increases.

You can sometimes use that to your advantage.
As long as the base fits the chamber, it’s sometimes possible to lengthen the base to shoulder measurement. Helps with the short ones, but can also actually reduce headspace to zero or a crush fit. Always good to double check before and after sizing.

Main thing is, if your working the die down looking for first contact at the shoulder, don’t be surprised of the measurement gets longer, before it gets shorter.

 

[Rsadams]

I have been told an AR can't fire if the bolts not fully closed.... Pull your bolt and move it in and out.... You will see that the firing pin can't poke through the bolt which would make it very hard to ignite a round.... That being said I trust no machine with my life unless I am forced to....

 

[ronemus]

I can size brass very consistently for my AR-based spacegun, but only after modifying the gas system (+2" port location for 24" or longer barrels). With a standard-length gas system I was getting obvious rim lifts and torn rims - no consistency and sizing only partially removed them. I also run an M16 bolt carrier that has a slug of stainless steel pinned in the back. Those measures delay extraction until the chamber pressure has dropped. The standard system works well with military loads, but I'm using slower powders with heavier bullets in 28" barrels, so it's no wonder there were issues.

 

[RegionRat]

I will agree with all of the above, plus add in the recommendation for owning or borrowing the 204R Go Gage to establish the chamber shoulder datum length.

I have gone through many bricks of that Fiocchi 40 VMax ammo in 204 and 223, as well as many WOA barrels. Every one of the WOA chambers and barrels have been awesome. Like your observations on the Fiocchi shoulder datum length, I saw lots in some years that were as short as yours, and some just a little better, but all were really too generous. My main gripe, would be the huge swings in velocity from year to year.

Still, those WOA uppers would make it work. I find split necks on somewhere from 1-2% with this Fiocchi. I anneal them and size them with no issues, and many times the shoulder is not bumped at all on the first or second reload cycle, however I measured my chamber and set my die based on a transfer from the gages.

Good Luck!

 

[jelenko]

If I were to load for a gas gun, I would use a GO headspacing gage and some shims to find out what my chamber measured, then set my dies to size .003 shorter.

Are there shims made that could be used for this? Otherwise, I'm thinking to use transparent scotch tape.

 

[Coyotefurharvester]

0.002 shim stock is easy to cut, I use it occasionally under a case in the shellholder to slightly move the shoulder on brass that has just a little tension when closing the bolt.

 

[SKWERLZ]

I have a WOA 204 upper. I have fired about 300 factory loaded Fiocchi. Im about to reload the once fired brass. I measured some random unfired factory loads and the once fired brass with the hornady tool. The once fired brass is all over the place?

I think I once read that the gas may need to be disconnected to get it to fire form consistently to get the measurement? Is that correct? Would it be worth the effort? Or is the data below enough to work with? I'm going for .003 bump so with that in mind i thought 1.553 or 1.552 might be a good starting point?

I guess my most important question is if there is not enough bump can it cause OOB kaboom in an AR? Or will it just not fire?

In contrast i have a 6 creed bolt gun and once fired hornady match is very consistent. 10 pieces were all +\- .0005. heck thats probably beyond the accuracy of me and the measuring tools.

View attachment 1347968

Thanks in advance!

Did you remove the primers before measuring them?

 

[Coyotefurharvester]

^^^^ great question, I always decap before any brass prep and just assume everyone would before measuring/die setup steps. Also wash AR brass after decap.

 

[jelenko]

And, check for any burrs from an ejector swipe. Why would I know that?

 

[Bronsin]

Are there shims made that could be used for this? Otherwise, I'm thinking to use transparent scotch tape.

You can make some from shim stock. Brass works well, and steel is fairly easy to cut at .001 and .002 thickness.

 

[jelenko]

Thanks. Bought some .001 brass shim stock on amazon.