Shoulder Bump

[22~308TargetShootr]

Loading for about a year now, and rounds are shooting well and I'm staying competitive in matches with my loads, but not really sure about this. I'm measuring a fired case with the Hornady headspace gauge or shoulder gauge and it's typically approximately 1.625, now I was taught to just try to bump back .001 to .002 in resizing. So I had everything set up and was getting cases coming out at 1.624 to 1.623 and thought Ok swell right on target. After seating I gave rounds another check just for kicks and then found the measurement to be 1.620 to 1.622, What the @!$& ? Am I doing something wrong or missing something ? Im turning out the overall length I want (measured to Ogive) and all, and rounds are shooting fine but am just perplexed as to the change.

C.D.

 

[antagonist]

Re: Shoulder Bump

Could be several things. It could be a measurement error. Digital calipers that can be rezeroed with the press of a button are a mixed blessing. A piece of dirt not wiped off before zeroing or removing and replacing the comparator jaws could make that much difference.

I don't know if you are crimping the cases intentionally or unintentionally, but it's very easy to crush the case with an improperly set up seater die. In most cases the seater die will have a crimping feature. If the cases aren't the same length they aren't going to give a consistent crimp. If it was set up to not crimp, but not backed off far enough a longer case could engage that feature, but without a cannelure it would probably try to push the whole neck down and can buckle the shoulder.

There are probably other possibilities, but I would measure any cases you have that you haven't seated a projectile in to see if they still measure what you expect. If they do, I would set up the seater die again with particular attention made to backing off the crimper at least a full turn or two. That seems like the easiest course of action.

 

[waste_knot]

Re: Shoulder Bump

And just so you know the Hornady gauge reads about 0.010 short with a 0.400 ring stub. The hornady is good for brass comparison but measure a 1.630 308 GO gauge and you will see. What does your bras measure when it is fired? As far as the brass changing it's probably from the tapered shoulder not being perfectly round or in the same plain as the comparator.
http://www.snipershide.com/forum/ubbthreads.php?ubb=showflat&Number=3330864#Post3330864

 

[Heman]

Re: Shoulder Bump

I have noticed the same thing with my brass where you do get some varience, however I usually don't worry about it. Last year I shott 2 2x1000yd matches where I had some brass that was formed from LC Match .30-06 to .280 using my Hornady Hyrdo forming die and the shoulders weren't completely crisp, along with brass that had been fire formed. Looking at the shot plots along with elvation records there was no appreciable difference in accuracy that I noticed shooting with coat and sling at 1000yds. I also tried it again last year in a 2x1000 F class match with my Palma rifle shooting fire formed vs. virgin brass. Again there was no appreciable difference between strings and in the last string I mixed 4 formed and 4 virgin and had 2 consecutive X's with 1 of each. So long as the brass chambers I wouldn't worry about it as the length differences you are talking about are literally splitting hairs.

 

[Paladin2147]

Re: Shoulder Bump

The way (amount) and what you lube with matters. Try - you'll see.

 

[22~308TargetShootr]

Re: Shoulder Bump

I was actually thinking could it have to due with the lube,.....

 

[Paladin2147]

Re: Shoulder Bump

Some of us have been down these roads before. Also, if your brass has some firings on it, its getting hard. That will also affect your bump. What I'm trying to tell you is your bump will move about some. Find a middle ground number and be happy. As long as your ammo is reliable, and your not hammering on the bolt to close it, its all good.