Newbie Question on Load Development for Gas Gun

[daved]

I've had a good deal of experience developing loads for precision bolt action rifles but have been very frustrated in developing a load for my newly acquired JP LRP-07. I expected that both shooting and loading for a gas gun would be more complex than for a bolt action rifle and that was actually one of the reasons I decided to buy the JP. But I'm finding I'm spending more of my time trying to develop a load rather than enjoying shooting the rifle. So any help on the following points or any not mentioned would be a help.

Specifics: 6.5CM, 22" barrel; COAL (Sinclair) 2.875"; H4350 powder; Berger 130 OTM bullets; CCI BR2 primers; desired MV range 2750-2850

For my bolt guns I determine powder charge by comparing 0.5 grain increment loads in my desired MV range and looking for the point where there is little vertical dispersion in the groups. I then use the method advocated by Eric Cortina to determine seating depth - find the "jam" depth, back off .020, and then start incrementally decreasing the seating depth by .003 to find a range of 2-3 small groups, then load to the higher end of that range. With the JP, groups are so inconsistent that there is no uniformity of results - one test session may identify several 0.3-.4 MOA groups. If I repeat the same test under same conditions a day or so later, all the groups have changed size!

So:
1) Will changes in lubrication and fouling of the reciprocating components over 25 - 50 rounds alter groups?
2) Should LD be done with gas off to eliminate variables or with the gas system tuned to the approximate MV of the groups being tested?
3) Does an LMOS allow the bolt to open before the bullet leaves the barrel, which is probably not a good thing in terms of small groups?

Thanks for any assistance.

Dave

 

[spife7980]

Yes, a gas gun that isn’t lubed will shoot different than it does lubed. That said the lube won’t disappear in 25-50 rounds, it’ll last a few hundred until you clean it and lube it again. If not a couple drips to moisten it will be just fine.
I think the change is due to requiring more gas to move a dry dirty gummed up bolt carrier than it does a clean one that offers little resistance. Just don’t let it get too dirty, clean it when you change your trucks oil.


I would not do development with the gas off. I would do a crude assessment so that you find something in the ballpark and get the gas set there accordingly and then get into the specific details. It’s an extra step but that’s what I’ve found in my limited gas gun experience.



The opening early would be more a function of the gas length, port size, block adjustment etc. and how that falls into the timing of all the pieces. If you’re gonna build it up light then get the rest of the bits to match it. A JP should come properly set up though so I wouldn’t worry about it too much.

 

[straightshooter1]

Don't know if any of this will be of help to you, but here it is anyway:

AR, Garand, M1A — Six Rules for Gas Gun Reloading « Daily Bulletin

SFC Lance Dement as featured in CMP's First Shot Online. The U.S. Army Marksmanship Unit (USAMU) has published a great series of reloading “how-to” articles on its Facebook Page.

bulletin.accurateshooter.com

6.5 Creedmoor AR Showdown

Why don’t more people use gas guns in the precision rifle world? I get asked that question a lot. Ma...

precisionrifleblog.com

 

[Gustav7]

How did you come to the conclusion of that bullet? Maybe its just not agreeing with your rifle?
I had consistency issues with Nosler 77gr CC in my AR.

How many rounds you have down the pipe?
Maybe try a different bullet first?

 

[daved]

Thanks for the input.

One thing I didn't mention initially is I use a mandrel to get about 004 bullet tension rather than the 002 I use with my bolt guns. HS -002 to 003, anneal and trim every firing.

Bullet choice was b/c I like Berger products and the 140 Hybrid supposedly doesn't feed well at longer than standard COAL - something about the length and angle the bullet has to take due to the compound ogive. I've also tried some Hornady 130 and 140 ELDM factory and reloads and get the same inconsistent results.

Total rounds are about 350 - enough to have settled in.

As far as lube, I usually put a couple drops off oil in the gas ports, wipe dirt off the surfaces of the bolt carrier and reapply lube each time I shoot, and single feed (except when checking feeding for an extended COAL). I'm trying to keep the platform as consistent as possible but I'm obviously missing something.

 

[HemiPowrd]

This may or may not help you, but I have the LRP-07 in 6.5 CM. I'm a newb reloader and started reloading specifically because I was shooting factory Hornady 140gr ELD-M ammo and sometimes shot really good groups with it and sometimes not so good groups. When I put a MagnetoSpeed on it I found out why as there was as much as a 75 fps spread with the factory ammo.

Since I knew there was good potential with the Hornady 140 my first goal was to just duplicate that round. What I ended up with are average groups of .625" with a point of impact that is very consistent. I'm not a great shooter so group size might be better with a better shooter behind the rifle. My loading doesn't meet your goal of 2750 to 2850 fps, but there is enough similarity to your load that it might give you a starting point.

I did ladder tests and seating depth tests to arrive at the following load.

Brass: Lapua small primer - mandrel sized to .261 inner diameter for .003 neck tension (not crimped). Brass is annealed every firing.
Bullet: Hornady 140gr ELD-M
Cartridge OAL: 2.810
Ogive length: 2.180 (as measured with a Hornady comparator). This gives a jump of approximately .062 to the lands.
Powder: H4350 40.3 gr. - gives 2663 fps average.
Primer: CCI 450

Obviously this is on a different rifle with a different bullet so feel free to ignore if it doesn't help.

 

[Gustav7]

Thanks for the input.

One thing I didn't mention initially is I use a mandrel to get about 004 bullet tension rather than the 002 I use with my bolt guns. HS -002 to 003, anneal and trim every firing.

Bullet choice was b/c I like Berger products and the 140 Hybrid supposedly doesn't feed well at longer than standard COAL - something about the length and angle the bullet has to take due to the compound ogive. I've also tried some Hornady 130 and 140 ELDM factory and reloads and get the same inconsistent results.

Total rounds are about 350 - enough to have settled in.

As far as lube, I usually put a couple drops off oil in the gas ports, wipe dirt off the surfaces of the bolt carrier and reapply lube each time I shoot, and single feed (except when checking feeding for an extended COAL). I'm trying to keep the platform as consistent as possible but I'm obviously missing something.

I might try to back off the neck tension to .003, or even 0.002 with a light crimp. I just recently loaded up bulk ammo and my neck tension was pretty high and seating consistency went out the damn window lol. I went back to 0.002 tension and run a Lee crimp die now and that helped a lot. You also didn't mention what brass you're using? If you're using good heavy brass, 004 tension may be even stronger than you realize, exacerbating the issue.

I mean...I don't think that much attention to lube care is needed. I think the issue with that comes when you do load development with a pristine rifle, and then wonder why its throwing rounds after 500rds of suppressed shooting lol.

 

[daved]

I did ladder tests and seating depth tests to arrive at the following load.

Brass: Lapua small primer - mandrel sized to .261 inner diameter for .003 neck tension (not crimped). Brass is annealed every firing.
Bullet: Hornady 140gr ELD-M
Cartridge OAL: 2.810
Ogive length: 2.180 (as measured with a Hornady comparator). This gives a jump of approximately .062 to the lands.
Powder: H4350 40.3 gr. - gives 2663 fps average.
Primer: CCI 450

Obviously this is on a different rifle with a different bullet so feel free to ignore if it doesn't help.

Thanks, that's quite helpful!

You don't seem to have the inconsistency of reloads that I'm experiencing. Does your JP have the LMOS and standard SCS?

The max COAL/CBTO of your rifle with the H140ELDM is within 003 of mine, measured with a Sinclair COAL tool and Hornady comparator.

I also note that when I was using up some old components breaking in the rifle and fire-forming new brass, 40.0 of H4350 gave around 2660 FPS with the few 140ELDMs I loaded. Since I wasn't doing actual load development I didn't save any ES/SD data but looking back on the groups, they were actually pretty good. I think I'm going to try some more of the 140s - one advantage to them over the 130 OTM is that it's easier to keep the COAL in mag length range for a given jump compared to the 130 OTMs. Guess that's due to different positions of the ogive.

I might try to back off the neck tension to .003, or even 0.002 with a light crimp. I just recently loaded up bulk ammo and my neck tension was pretty high and seating consistency went out the damn window lol. I went back to 0.002 tension and run a Lee crimp die now and that helped a lot. You also didn't mention what brass you're using? If you're using good heavy brass, 004 tension may be even stronger than you realize, exacerbating the issue.

I've been using Hornady brass but have some fire formed Lapua I'll be loading next. I will try my bolt gun BT 001-002 and see if I run into any set back issue.

Thanks for the comments---

 

[HemiPowrd]

Thanks, that's quite helpful!

You don't seem to have the inconsistency of reloads that I'm experiencing. Does your JP have the LMOS and standard SCS?

The max COAL/CBTO of your rifle with the H140ELDM is within 003 of mine, measured with a Sinclair COAL tool and Hornady comparator.

I also note that when I was using up some old components breaking in the rifle and fire-forming new brass, 40.0 of H4350 gave around 2660 FPS with the few 140ELDMs I loaded. Since I wasn't doing actual load development I didn't save any ES/SD data but looking back on the groups, they were actually pretty good. I think I'm going to try some more of the 140s - one advantage to them over the 130 OTM is that it's easier to keep the COAL in mag length range for a given jump compared to the 130 OTMs. Guess that's due to different positions of the ogive.

I've been using Hornady brass but have some fire formed Lapua I'll be loading next. I will try my bolt gun BT 001-002 and see if I run into any set back issue.

Thanks for the comments---

I did have the LMOS and SCS in mine until recently. I added a suppressor and changed to the JP heavy buffer and bolt carrier. The only thing that changed was point of impact so I just had to rezero and I'm good.

I was wondering if your jump was different due to the Berger 130 and it sounds like it is. My bullet seating depth tests showed similar impact points with jumps from .062 to .077 using the Hornady 140 ELD so I went with the shortest jump in that range in the hopes that as my rifle lands wear and the jump increases I won't have to adjust my load for a while. I did buy one Knights Armament magazine since it allows COAL up to 2.865 (or something like that) so I can seat longer in the future if I need to or if I decide to try a different bullet that needs it.

One thing that really helped my SD/ES was using a mandrel to size the inside of the case neck diameter. I noticed the pressure needed to seat a bullet was much more consistent and brought my SD/ES down a good bit. I use a .261 Sinclair mandrel and expanding die. Previously I was just using Redding neck bushings to size the neck.

Good luck!

 

[HemiPowrd]

One other thing: If you're going to use expensive brass you'll want to put some velcro (loop side) over the brass deflector to keep it from damaging the brass as it is ejected. The JP deflector has a very sharp edge and was leaving dents in the shoulder area and also significantly denting the case mouth on some cases.