7PRC sizing inconsistency

[Charlie-NY]

I'm working with a new 7PRC with a Bartlein carbon wrapped bbl and new Peterson brass. The brass was loaded with various bullets and loads, then fired. I set my Redding Type S FL die to give about .002" shoulder set back. I'm using Imperial Wax and avoided getting it on the shoulders.

The first thing I noticed was that some brass needed more force than others to fully resize. At first, I thought that the wax was applied inconsistently but then thought that the different loads caused the brass to expand differently. None of the loads were hot. I ran each piece of brass into the die twice and gave a second of dwell time at the top of each stroke.

Everything looked good until I measured the base to datum line using the Hornady tool. I found that there was a variation of .003" between the shortest and longest cases. What caused this and what is the best way to avoid it?

 

[nakneker]

I'm working with a new 7PRC with a Bartlein carbon wrapped bbl and new Peterson brass. The brass was loaded with various bullets and loads, then fired. I set my Redding Type S FL die to give about .002" shoulder set back. I'm using Imperial Wax and avoided getting it on the shoulders.

The first thing I noticed was that some brass needed more force than others to fully resize. At first, I thought that the wax was applied inconsistently but then thought that the different loads caused the brass to expand differently. None of the loads were hot. I ran each piece of brass into the die twice and gave a second of dwell time at the top of each stroke.

Everything looked good until I measured the base to datum line using the Hornady tool. I found that there was a variation of .003" between the shortest and longest cases. What caused this and what is the best way to avoid it?

It’s worth checking new brass for uniform length. There are times I’ll trim new brass to get it uniform is there is a lot of variation in length, it’s nice to start with brass that’s the same length.

I’ve gone to using mandrels, I get my best results using a mandrel to set neck tension instead of pulling the sizer button back through the case neck. Just a couple of thoughts that came to mind after read your post.

 

[358WCF]

Is this the 1st firing on all cases? "Different loads, none hot" sounds to be a likely reason why there's a difference in sizing force... also could be why the headspace varies.

Maybe you shouldn't set shoulders back .002" until the bolt gets a bit harder to close after 2 or 3 full charge firings. That way the cases will be formed to the chamber better than with varying amounts of pressure due to different mild loads.

 

[Charlie-NY]

It’s worth checking new brass for uniform length. There are times I’ll trim new brass to get it uniform is there is a lot of variation in length, it’s nice to start with brass that’s the same length.

I’ve gone to using mandrels, I get my best results using a mandrel to set neck tension instead of pulling the sizer button back through the case neck. Just a couple of thoughts that came to mind after read your post.

I don't bother with overall case length until the cases have been fired at least twice. I didn't mention it but there is no expander button in my Type S dies. I've recently started using a 21st Centry expander in my cases.

 

[Charlie-NY]

Is this the 1st firing on all cases? "Different loads, none hot" sounds to be a likely reason why there's a difference in sizing force... also could be why the headspace varies.

Maybe you shouldn't set shoulders back .002" until the bolt gets a bit harder to close after 2 or 3 full charge firings. That way the cases will be formed to the chamber better than with varying amounts of pressure due to different mild loads.

Good points, however, I fired 5 cases with reasonably heavy loads twice to determine a starting point for shoulder set-back adjustment. I also used one of them to make my "modified case" for the Hornady tool.

 

[Ledd Slinger]

I wouldn't worry too much about it until you have about 3 firings on the brass. Using different powders absolutely will expand cases differently because the pressures are different. Different bullets with the same powder will also produce a similar variance on initial fireforming. Just set the headspace on your die to -002" from the longest case. Re-check them after 2nd and 3rd firings and adjust headspace on the die again as necessary.

If you are 3 firings in and still having issues, it's probably the brass

 

[nakneker]

I don't bother with overall case length until the cases have been fired at least twice. I didn't mention it but there is no expander button in my Type S dies. I've recently started using a 21st Centry expander in my cases.

I’m using the same, not sure what the issue is, hope you find it soon.

 

[Rank Amateur]

I'll concur with previous posts suggesting that the brass may not have been fully expanded to your chamber size with your initial firings. One way to determine if it is the die/sizing technique causing inconsistencies is to measure to the shoulder datum after firing and BEFORE sizing. The question you're trying to answer is whether the brass is being fire formed to your chamber. If it is, then the length to datum will be pretty consistent. Then, resize and compare several cases (measurements before and after sizing).

You may also consider whether you have adequate lube (I'm presuming you do, but you mentioned a tactile difference in effort needed to size the brass). A lot of "drag" on the walls of the case body can subtly shorten the measurement to the shoulder datum. Take note if this happens again to determine if the cases that are shorter after sizing were also those that required more effort to size.

 

[Charlie-NY]

I'll concur with previous posts suggesting that the brass may not have been fully expanded to your chamber size with your initial firings. One way to determine if it is the die/sizing technique causing inconsistencies is to measure to the shoulder datum after firing and BEFORE sizing. The question you're trying to answer is whether the brass is being fire formed to your chamber. If it is, then the length to datum will be pretty consistent. Then, resize and compare several cases (measurements before and after sizing).

You may also consider whether you have adequate lube (I'm presuming you do, but you mentioned a tactile difference in effort needed to size the brass). A lot of "drag" on the walls of the case body can subtly shorten the measurement to the shoulder datum. Take note if this happens again to determine if the cases that are shorter after sizing were also those that required more effort to size.

Twice fired cases measured a consistent 1.889" IIRC. I was looking for a sized case @ 1.887" but noticed the varied dimensions after sizing. Once I get all the brass fired twice, I'll see if things settle down. Generally speaking, I have never noticed that using more or less Imperial Wax would alter the results. I'll try to more consistently apply the wax and see if that helps.

 

[Ledd Slinger]

I personally would never use Peterson brass again. I had too may issues. I'll leave it at that