How to set head space on cartridge that headspace on mouth?

[BTSniper]

Picked up one of Dean's 50 Thumper barrels and I'm learning more about cartridges that head space off the mouth of the case. I'm not new to this as I used to have a 44AMP that also head spaced off the mouth and required close tolerance on case COL length.

My question is how much space or tolerances do we allow for when setting the head space for a cartridge that head spaces off the mouth?

Here is what I have..... 50 Thumper (500-375 Ruger, basically a straight wall case) brass with a COL of 2.65" this is new unfired brass. I FL sized them all the way to the base in a 500 S&W die then trimmed them all to same exact length of 2.652. To set the head space of this wildcat I placed a .002 thick piece of tape on the bottom of a case and used it to set the head space with.

Now after firing the full power round the case measures 2.645, after resizing again all the way to the base with the 500 S&W FL die the case has grown/lengthened to 2.660 and of course my head space is too tight now, not allowing the case to chamber.

Maybe it is just the new brass "settling in" or maybe my head space is too tight? I suppose I can do one of two things...... trim the brass a wee bit shorter or...... back the head space out ever so slightly?

So my question is again, how tight on the tolerance should a case like this have? and anyone have any thoughts or knowledge?

Thanks

BT

[Three44s]

I have zero experience with this combo ........ but you asked for thoughts:

I think you have a chain of events ..... you correctly identify your brass as new ..... that may be a factor. Also, you are FL each time ..... Can you partial size to the point of having enough neck tension but not leave as much free space around the exterior of the about to be fired casing?

Naturally, you'll need your chamber and brass as dry as possible.

With some cartridges, I'll seat the bullet out to touch the lands in order to keep the base against the bolt face for the purposes of fire forming. You would have to back off powder charges accordingly.

I realize you have formed your brass already but as a stop gap until you get your brass hardened by a few firings or otherwise figure this thing out.

I don't think just trimming your brass over and over will solve this ..... you'll just keep backing away from the problem. Some trimming may have to be done as a part of a comprehensive solution.

Lastly, what about contacting the source for your barrel and ask them for advice?

When I get in a real jam, I'll take just a very few cases and run them out over and over ....... this way you have not messed up so many until you find out what's going on. When you have solved your problem, you'll still have enough left to work with instead of having to try and find more ........ trying times for us gun nuts these days.

Some food for thought .........

Best regards and good luck!

Three 44s

[BillPa]

Now after firing the full power round the case measures 2.645, after resizing again all the way to the base with the 500 S&W FL die the case has grown/lengthened to 2.660 and of course my head space is too tight now, not allowing the case to chamber.

BT

I'd say your FL die is oversizing the brass creating a excessive head clearance issue, not a headspace issue. When FL sizing the brass can't move back(shellholder) or out (die walls) so it moves forward lengthening a case and the more the body is sized the more a case lengthens.

I'll bet if you back off the die then size a case an 1/8" at a time you can watch it grow like a Chia Pet. You have two choices, trim to length after FL sizing or have that or another die honed to minimum resize your brass.

Bill

[devildogandboy]

have you asked Dean, i'm sure he would give you the answers you seek.

Bruce

[scope eye]

OK Here is the low down, you do not full length resize straight wall cases after every firing, in fact you hardly ever have to nor should you, they only time you do is if the case is "DRAGGING"
when chambering or during extraction, the reason it shrank is because it formed to the chamber walls, that will only happen once, then the next firing it will grow to it's correct headspace,
and stay there from then on, all you need to do is open the mouth of the case to get the bullet in and crimp that's all, to deprime use a punch or a depriming die, the cases on a straight wall do not grow like a bottle neck, since there is little to no stress on the case itself, you will be surprised on how many firings you will get without any prep work or maintenance, hope this clears this up feel free to follow up with any other questions.

Dean

PS: as far as the case being to short now, just load it up and fire it will grow to it's proper length and stay there.

[BTSniper]

I think I may have it figured out. After I completed all the steps I followed the first time around I was within .002-.004 of my starting COL length of 2.652 and everything chambers.

Here is what I did with the once fired brass from my chamber

1. FL size to the base with the 500 S&W die. This die is about .522 in diameter.

2. size the top .5" of the case down in the taper crimp die. This was necessary do to the Hornady 375 basic brass was only about .011 think at the mouth, with the standard 500 S&W FL sizing die the mouth of the 50 thumper case had not been reduced enough to sufficiently grasp the .500 bullet.

3. Expand the top .5" of the mouth back out with the 500 S&W expander die, this provided the correct amount of grip on the bullet.

Good to go!

It was just a short learning curve for me with the new caliber. I had a combination of things that lead to the longer COL of the brass, 1. the primer was protruding about .002 (simply seat it deeper next time) 2. I took the measurement after I FL sized it and before I completed step 2 and 3 above. After the spent primer was ejected and I sized down the mouth I was back to within .002-.004 and everything chambered as it should. I imagine as I seat the bullet it might just take care of that last .002.

For sure a fun cartridge with a lot of potential. I'll take the lead sled out next time and see what can really be done!

Swage on!

BT

[sixonetonoffun]

I put one.down the pipe for giggles. Not quite done with it but had to try it once. I did have to use a mallet to eject.the shell casing. Flattened the primer good. Didn't crater.it and only left a slight mark from the.bolt face. I'm going to recheck the head space before I load any more rounds for it. I went a little overboard with my enthusiasm and made my first round 97.5 gr of ramshot tac, cci 250 primer pushing a sierra 400gr JSP it was.over 80 today but I will definitely take the load down a little as I am not sure this particular bullet can handle this kind of pressure and velocity. Even with ear plugs this thing really barks.

[scope eye]

That is awesome I love it, I am sure with 90gr of Tac you will be OK, I have not shot heavier than 300gr bullets,
a hundred grains of powder will have a tendency to do that, that barking thing I am talking about.

Dean

[Blitzfike]

<snip>
Dean

PS: as far as the case being to short now, just load it up and fire it will grow to it's proper length and stay there.

I guess from this that I'm worrying too much about the .008 difference in new new brass lengths. The 10 or so rounds I've shot so far haven't given me any indication of case length problems, but they may have all been of the length case I set head-space with. Rimless straight wall cases are new for me in the development stage. Even an OLD dog can learn new tricks sometimes... Jim