RPR 6mm CM project and reports

[Newton]

New RPR owner here. Thought I would share my results and progress as it comes along. Sorry for the long first post. Figured I would just spill it all off the getgo. I am sure my progress reports will be shorter.....

I was somewhat concerned about what my throat might be after I started seeing a few threads on here and other places indicating that some RPR in the 6mm had issues. I first tested my rifle using Nosler 105hpbt custom comp bullets. I did not have an OAL gauge at the time(I do now, and will re-test) so I just did the close the bolt on a case and bullet test, and it came out looking like I had a long throat actually. I was able to seat them to a 2.790" OAL and have a bit of a jump. That was the longest I felt like seating them because at that length they don't have very much in the case neck.

I did shoot a few last week using some IMR4451 just to see how the gun felt. I did not shoot for accuracy, but I was impressed with how it did shoot. Then I went to the local Cabela's to get some Reloder 16 and saw they had Hornady bullets on sale. So I picked up a box of 105gr hpbt and a box of 108gr eldm.

I wanted to see how the 108gr eldm fit in the gun so I did the close the bolt test again and saw it was not pushing the bullet into the case(unless I seated it out really far). So I marked it up with some sharpie and tested again. This time the bullet stuck in the gun. When I knocked it out, I saw the lands had rubbed the sharpie away. So I repeated this test a few times and sure enough, the lands would rub the sharpie away almost down to the case neck.

However, when I wiped the sharpie away, there were no physical marks on the bullet itself. So that really leads me to believe that the whole thing with short throats being reported is that its throats that are just not reamed out large enough in diameter, not that they are not long enough in length/depth. I know others have come to this conclusion, but I had not heard of anyone with a story like mine yet.

On the bright side, it does seem like this would be excellent for accuracy as it seems like when loading a round the throat is going to center those 108gr eldm's perfectly and hold them there. It makes sense to me now why so many people have said they have got great accuracy from the match ammo hornady makes.

I have not actually started testing them yet because I have so many of the Noslers to shoot. Figured I would 'break in' my gun and get used to it with the lower priced bullets, then do some testing with the Hornady's.

Nosler does not give any information on the 105gr custom comp bullets in the 6mm creedmoor load data. But, when I compared the hornady 105gr hpbt I see that they are almost the same length(some of them are actually, just depending on how the tips are). The base to ogive measurement is different. I figured I could probably use Hornady load data for a good starting point.

So I did load and shoot some yesterday. I loaded up some at 40gr, 40.5gr, and 41gr. I loaded them to a 2.790" OAL, which put me at a 2.222" BTOL. What surprised me is that the velocities are way different than what Hornady lists. I figured they might be a little different, but not as much as they were. Hornady's test gun is also a RPR, I am using Hornady brass, and CCI LR primers.

40gr RE16 averaged 2866fps. 40.5gr RE16 averaged 2904fps. 41gr RE16 averaged 2950fps.

I then loaded some up at 41.2gr and they averaged 2960fps. I am going to redo the tests, I am wondering if it just wasn't light enough. I have always gotten better readings on cloudy days, but maybe it was just too cloudy. The velocities just seem to be way low compared to what Hornady lists. I will say that I sometimes wonder about Hornady's data though. They group 4 different bullets in the same list, ranging from 105gr to 110gr. I have to wonder if their data isn't more for the heavier bullets. Their max for RE16 is 41.1gr at 3050fps. That is almost a 100fps difference than what I got.

Good news, if you consider it that, is that all the charges were well under MOA. The testing was all done at 100 yards. I did load up some with 41.3gr(the 41.2gr showed zero sign of any pressure) and shot some steel at 200yd and 250yd for fun. I did not chrony them because it was definitely too dark to do it by then. The gun is a blast to shoot. I did shoot a 1.5" 5 shot group on the 200yd gong. Some of the groups I shot would have been one hole if they had not strung vertically. I really think that my be my form behind the gun.

I should say that I am not looking for speed, I am more interested in accuracy. I just wonder if I should move on up in the charge weight since I am not seeing any pressure.

I just got in my OAL gauge and a modified case, so I will be able to get some more accurate measurements on the throat.

 

[Newton]

Have a quick update. I found myself with a little bit of time yesterday afternoon so I tried out the OAL guage. It did not come out like I had expected to tell the truth. For one thing, it is hard to use in the RPR action. I tried it several times and was coming up with an OAL of 2.790". Which is what I have been seating them at. I can only assume that the original bullet I used must have been slightly different sized, or just performed differently with the different test. I honestly have no clue right now and figured it was going to take some time messing around with it all to get a good feel for it. I did rack one round I had made up, then extracted it, and could see where the lands were slightly kissing it - kind of like others have shown and what I saw when testing the 108gr eldm's.

But, the weather was perfect out so I decided to test some more loads while I had the chance. I tested the 41.3gr of RE16 again, then bumped up to 41.5gr, and ended with 41.7gr.

41.3gr avg was 2950fps
41.5gr avg was 2994fps
41.7gr avg was 3016fps

There was way more light than the day before, so I figure that's why my numbers were a little different. I was seeing no pressure signs again, but I did notice that starting with the 41.5gr load, the primers started having a raised bump on their surface. I looked at my bolt face and there is a little imperfection that matches up with what I see. So I am going to assume that while they are not flattening around the edges or anything, they are starting to see some pressures. There is no hard bolt lift or sticky extraction at all, and they are relatively round at the edges still.

The good thing about it is the 41.7gr load shoots great. I had enough time to shoot one 5 shot group at a gong at 250yd's and got a .5 MOA group with it(according to the "on target" software, the group was .6" wide and 1.3" tall). I can only imagine the more I shoot the better it will break in. My next outing I will set up a paper target at 200yd and test some loads.

Would you all consider the 105gr Nosler at 3016fps a medium or warmer load?

 

[47WillysGuy]

My guess is that 105 gr bullets at 3016 FPS would be a medium load, but it’s a guess. I chrono’d my 6CM at 3060 FPS, but we use different components.

I don’t like to load to maximum pressure.
- if air temp is above my final test load, chamber pressure will rise, which could be catastrophic.
- max pressure equals shorter barrel life.

A F-Open competition 6CM barrel loaded to max most likely will be changed out at 1000 rounds. Or maybe even earlier. I have 1300 rounds on my 6 CM and it shoots a lot of 10’s but not many X’s. That’s a sign it’s about finished.

Your cases will tell you when you reach the upper limits:
- bolt swipe on caseheads
- hard bolt lift
- primer flattening on heavier loads not seen on light loads
- chronograph that shows an awesome velocity

My process for finding an accurate load is:
-buy 100 Lapua cases or a favorite brand
-sort cases lightest to the heaviest
-79 for competition(lightest)
-21 for testing(heaviest) these never will used in competition
-load 7 increasing increments of 3 ea powder charges
-expend the 21 rounds at the range watching for the upper limit signs
-then I measure the expended cases paying particular attention to the case body diameter about 0.120” from the casehead.

My standard is if the difference between the very heaviest load fired and lightest load fired is 0.001”, I back off that powder charge.

Your right about the long post, whew! LOL!
And Work up to max loads carefully.
Also, there’s a lot steps I’ve left out. Happy shooting!