Inconstancy in case length for shoulder bump

[jh222503030]

Here’s the scoop, I’ve been reloading mostly pistol or for my AR for a couple of years now. In the last year I’ve purchased a Kimber SuperAmerican in 300WSM. Now I’m trying to get consistency down to an art with reloading for the 300. I’ve purchased close to 500 bucks in tools that I have seen suggested here on the forum. I’m actually getting pretty anal of how I reload for the Kimber.

But…..

I’m getting different spring back measurements in the Winchester “new brass” that I’ve been using for my test loads. My cartridge lengths (fire formed) run from 1.728 to 1.742 (out of 18 rounds fired)to the datum. How am I going to get a .002-.003 shoulder bump with so many different lengths?

I know I can fire perhaps 5 rounds before doing a FLS of the brass and just do neck sizing. Yet with the different fire formed sizes, doesn’t that affect the OAL? Or should I just worry about the length from the case head to the ojive?

TIA

 

[SPAK]

Re: Inconstancy in case length for shoulder bump

How are you measuring to the datum?? Are you using a bump gauge?

 

[jh222503030]

Re: Inconstancy in case length for shoulder bump

Yes to both, using Sinclair’s bump gauge. This was how I found out that the measurements to the datum were not consistent. I knew that there would be some spring back, but the spread that I was getting seem to be abnormal.
Of course on my previous reloading for the pistol and AR I never was concerned about anything other than proper charges and OAL.
With the Kimber, I want to do it right the first time, and every time there after.

 

[SPAK]

Re: Inconstancy in case length for shoulder bump

.014" difference seems like a lot....When you chamber your empty once fired brass, do you feel any resistance with the longer brass???

Also, make sure that you deprimed before measuring. Sometimes the primer can interfere with your reading.

If your loads are really light, it might take another firing or two for your brass to fully form to your chamber.

 

[427Cobra]

Re: Inconstancy in case length for shoulder bump

Decap the brass and measure with the HS Gauge, primer flash can skew the fired reading, most if not all your fired brass should read the same, then set up your FL die, do not put any lube on the shoulder, just the neck and body

 

[jh222503030]

Re: Inconstancy in case length for shoulder bump

I agree 0.14 seems excessive. I was thinking perhaps it would not be much more than a few thousands off between cases and that thinking were like .001-.003.

I do deprime, and was getting the difference, even unprimed there was a difference. What’s funny I have some Winchester Nickel brass and there is very little difference in the fire-formed brass.

The loads were in 65.2-66.2 on the first 18 rounds and 66.4-67.4 on the second 18 rounds. I do three shot groups, using H4350 Ext.

On my factory Federal ammo I had more consistency in case head to datum measurements. Had 8 at 1.726; 2 at 1.728; 1 at 1.729; 1 at 1.734; 1 at 1.735; 2 at 1.726 and 3 at 1.737. This was regular brass, and not the nickel stuff.
Re-chambering the fire-formed brass……there might be a slight resistance.

I thought that perhaps I was holding the brass incorrectly in the calibers with the bump gage on, so I put the other Comparator Insert on the other holder (to have more of a flat surface) and re-measured. I got the same readings.

Tks all for the help.

 

[LawnMM]

Re: Inconstancy in case length for shoulder bump

Measure a decent sample of fired cases, take the average, and set the bump for .002 back from the average. The die is set, so anything longer should get bumped back to where you set it, anything shorter shouldn't get bumped at all.

The longer cases will get more of a bump than the short ones but the end result should be the same, a consistent measurement on sized cases.