Experiences with high overbore cartridges and powder bridging

[Mike Matteson]

Looking for any information or experiences on powder bridging within a cartridge and pressure spikes. We ve built 2-3 6mm-280 AI s and in testing found Retumbo to be a little quick burning ( pressure signs before we hit anticipated velocities) and then moved to R33 and US869. Our velocities were higher without pressure signs at 80 degree temperature range. (3400 FPS range with 27-30" barrels)

Fast forward and we ve seen high pressure with same R33 loading at 40 degrees. Same batch of loads. Chose R33 over 869 as thought it would be less temperature sensitive, and logic would anticipate LESS pressure in colder temps.

So in trying to figure out what's happening came upon the powder bridging notion and read an old post on this forum that I think Fiftydriver wrote saying that stick powders in small caliber overbore cases can cause powder/bridging and erratic pressures and that ball powders are less prone to such.

Have any of you experienced such and was there a viable solution? R33 is a large kernel stick powder and I can see where this makes theoretical sense. We re burning 64-65 grs in the case behind 108-110 gr bullets.

Interested in any experiences. Thx

Interesting. I am having a 6mm-280AI being built presently. I see that there is people that are ahead of me. I am using a Rem 700 action, a Bartlein Barrel Metal Grade, 400MOEDBB heavy Palma, twist rate of 7-1 cut to 30". Looking and have Hammer 100gr bullets and Berger bullets on hand. Order out 500 Peterson 280AI brass, and have others in different case sizing presently. I generally use H powders, and mostly H4350, I trying to get my hands on H4831, H1000, and Varget. I generally stay away from Reloaders, and IMR powders, and few other do to extreme temperature variances. I generally use fed 210 primers with powders loads up to and including 75.5grs of powder, but these are in 300 mag area.
Any info that you are willing to share would be welcome.

 

[fmuguira]

I would suggest H1000 and Retumbo as workable but you'll still have a lot of case space left even when you ve reached max pressure. Start low and do your own work ups; preferably with a chronograph assisting you too. 57 grs of H1000 will be getting warm/hot depending on your rifle and 59/60 of Retumbo will do like wise. That's with a 105-108 grain projectile. We re running 62-63 grs of R33 and its max in our rifles, gives an extra 50-75 over the single base Hodgkin powders. H869 you can approach 66-67 grs depending again on your rifle.

As mentioned earlier these are max range loads in our rifles, with our free bore, etc. Start lower and be cautious and gradual. I always remind myself and others that the last 50-100 FPS really makes no measurable performance different; unless you consider loose primer pockets, beaten up brass, etc a gain !!

Good luck

 

[esshup]

The "go to" powder for the 7mm Allen Magnum was WC872 (a flattened ball powder) with the heavier bullets (175-200 grain) and no bridging issues with that, BUT I noticed that it was VERY temp sensitive, as in the colder it got the slower it went. I didn't run it over the chronograph, but I zero'd the gun at 6,000' elevation and 40°F, then 2 days later during the hunt the temps dropped to below zero. I had shot the gun from 300 yd to 1400 yd to develop a drop chart at 6,000'/40°F. At -8°F and 5,000' elevation the bullet, at 700 yds, was impacting 2 moa low. That's why I went away from the WC872.

I first started with RL-33, but couldn't seem to get the velocity without pressure signs with a 180 or 195g bullet, so I switched to RL-50. Velocity picked up with out showing signs of pressure 195g bullet and RL-33, 3084 fps was what I could get without having signs of pressure. With the same bullet and RL-50, I was able to push it 3250 fps before I saw pressure signs, 3225 was the accuracy node.

I can't give any info on bridging issues with the RL-50, I haven't shot enough rounds to come to a conclusion, less than 20. Kirby said he didn't have any bridging issues with the RL-33 in the 7mm AM, but did with RL-50.

 

[Lefty7mmstw]

I'm running h50BMG pushing a 175eld-x in my 7rum with no issues with bridging. I really don't use Alliant powder slower than rl25/26 in 7mm's. The performance on rl33 and rl50 was a bit flaky; a good load in warm weather might go to pot if hot or cold weather came up, even though the speed was close. I really don't think there's enough load on the powder to keep it consistent with a lot of 7mm pills- 195's may be a bit different though on that. I do, however, run either rl33 or rl50 in my 300rum's with 225 hdy hpbt pills. I'm a couple percent down from top pressure but still netting 3,000 fps.

 

[Bob Wright]

Most 280AI loads with RE23 and a FED GMM215M are extremely predictable with good ES/SD. A 143 GR bullet responds well.
The double base powders seem to perform well in the 280 or 280AI. RE16 OR 23 might help you and test with a mag primer and non mag primer. You have quite the wildcat. Good luck.

 

[Mike Matteson]

I would suggest H1000 and Retumbo as workable but you'll still have a lot of case space left even when you ve reached max pressure. Start low and do your own work ups; preferably with a chronograph assisting you too. 57 grs of H1000 will be getting warm/hot depending on your rifle and 59/60 of Retumbo will do like wise. That's with a 105-108 grain projectile. We re running 62-63 grs of R33 and its max in our rifles, gives an extra 50-75 over the single base Hodgkin powders. H869 you can approach 66-67 grs depending again on your rifle.

As mentioned earlier these are max range loads in our rifles, with our free bore, etc. Start lower and be cautious and gradual. I always remind myself and others that the last 50-100 FPS really makes no measurable performance different; unless you consider loose primer pockets, beaten up brass, etc a gain !!

Good luck

Thanks for the info. That is about what my thinking is on loading the 6mm-280IA. It was my thinking that changed me from a 6mm-06IA to the 280IA concerned about having enough case to use the slower powders. It appears that I have more than enough. I have a high end chronograph, and I use in load development. I don't take off with the high on powder ever. Learn that a long time ago. Have you tried the Hammer Hunter 100gr bullets by chance? I don't have my rifle yet, but do have the bullets. I am very impressed with them. As I noted earlier that I am using a 30" barrel, because of the slower burning powders. I generally use Large Rifle Primers, and not Mag Primers. Being the loads are under 75 grains. Don't get excited about the 75 grains, that level of powders is when I start to look at mag primers. It looks like we will be well under that. The other is I am waiting on powders, brass cases from Peterson brass in 280IA. All the case will be volume weighted, and necks turned. Presently upgrading, and updating my reloading equipment at the same time as having this rifle built.

I will keep you posted when I start building loads for it. It problem be 6 months or better yet.

Thanks and good luck to you too.

 

[stinkyduck13]

This.
Plus , light neck tension may allow the bullets to move to soon. Producing a 2nd pressure spike?

View attachment 231784

I was thinking the same, maybe a Lee factory crimp die?

 

[243winxb]

https://apps.dtic.mil/docs/citations/ADA341390

When the powder doesnt work with the other components, strange thing can happen.

A crimp may make the difference?


To avoid Delayed Ignition, use a mag primer, seat bullet deeper into the case, use a bulky powder that better fill the case. Increase bullet pull with a crimp, as a last resort. Crimps may hurt accuracy. Doing your own testing is the only way to know.

 

[MagnumManiac]

Also, might the 40 degree shoulder on the AI case contribute to the problem? Or, fairly deep bullet seating due to only .050 free bore?

The 40° shoulder should eliminate this issue, the powder column stays in the case.
I think your issue my be a cold weld condition…I had this issue myself a long time ago.
I run powdered graphite in my necks and also HBN coat my ELR/LR bullets, helped reduce my Agg quite a bit.

Cheers.