Compressed Loads

[MSLRHunter]

I have a question about compressed loads. I have never loaded any compressed loads, but I am trying some Retumbo powder in my 300 win mag with some Berger 190 and 210 VLD's. The data provided by Berger shows a max of 82.9 grains at 108% of case capacity for the 190 grain loaded to a COAL of 3.340 with a velocity of 2998fps. These bullets will be seated to a COAL of 3.558 in my rifle for the 190 grain bullet. How will the increased case capacity due to my longer COAL affect pressure and therefore velocity? Logically it would seem that the velocity may be a bit lower, but like I said, I have never run into this situation before.

 

[Gunpoor]

I would answer your question if only I can find what I did with that crystal ball. Oh well, in the mean time you could just shoot them over a chronograph and see what that tells you. In theory a longer COAL should reduce chamber pressure and therefore reduce velocity, that is unless you encroach the barrel lands which will then negate that theory by increasing chamber pressure. Why don't you shoot your loads and post the results and then let everyone dog pile you with responses? In large capacity cartridges the slow burning powders such as your Retumbo are nice because you will hardly be able to stuff enough propellent in them to cause dangerous pressure situations, but you need to work up slowly to find the sweet spot for accuracy. Good luck with you load developement!

 

[Moman]

I think like Gunpoor said, you'll have to watch for pressure signs as you approach the lands. I would definately start low and watch very closely for pressure signs as you work your way up.

 

[Fitch]

I have a question about compressed loads. I have never loaded any compressed loads, but I am trying some Retumbo powder in my 300 win mag with some Berger 190 and 210 VLD's. The data provided by Berger shows a max of 82.9 grains at 108% of case capacity for the 190 grain loaded to a COAL of 3.340 with a velocity of 2998fps.

This is one of the kinds of "what-if" questions that QuickLOAD is useful to study.

Modeling the Berger load in QL assuming Winchester brass, 26" barrel, and SAAMI chamber size (because I have no H2O capacity data), the resulting prediction is 109% fill (which might end up being 108% with actual water grain capacity), 3,021 fps, and 54,219 psi vs a max pressure of 62,336. From what I can see, if you can stuff it in there with out bulging the case so it won't fit in the chamber, Retumbo won't go over pressure. 109% is a lot of compression.

These bullets will be seated to a COAL of 3.558 in my rifle for the 190 grain bullet. How will the increased case capacity due to my longer COAL affect pressure and therefore velocity? Logically it would seem that the velocity may be a bit lower, but like I said, I have never run into this situation before.

I can estimate that.

Important assumption: You have at least 0.020" clearance from the lands. If you are touching the lands all bets are off and pressure will be close to 7,200 psi higher.

With that assumption in mind, the same model with just the COAL increased to 3.558" and everything else lef6t the same, QuickLOAD prdicts 103.9% fill, 2,936 fps, 48,691 psi. On that basis, the increased case volume caused by having less of it filled by bullet can be used for more powder with out going over pressure (a feature of a long throated chamber with a long bullet). Going for equal pressure to the 3.34 inch load, QuickLOAD says it would take 85.5g of Retumbo yielding 3,034 fps, but I'd work up to that. I never load to a QuickLOAD predicted max load with out backing off 10% and working up to it taking chronograph readings and inspecting brass. (some times I work up with one shot per charge in 1 grain increments looking for pressure signs and pull all the bullets after I see the first sign whatever it is (sticky bolt, polished case base, flattened primer, etc.).

I'd work up to the first load from Berger too.

The high percentage fill is an indication that the powder is marginally too slow for the bullet weight in the cartridge so I asked QL for a propellant table (load map) listing suitable powders in order of decreasing MV at the same pressure. Other powders to consider, all predicted to have higher muzzle velocity than Retumbo, listed top to bottom in order of decreasing MV:

N570
Ramshot Magnum (Big Boy)
RL25
N560
IMR7828
IMR7828SSC
Accurate MagPRO
H1000
RL22
Winchester WXR

Fitch

 

[MontanaRifleman]

It will depend on what brass you use, how you size it and where you seat the bullets.

As others have said, start low and work up carefully. Retumbo is a fairly forgiving powder being as slow as it is, but always carefully and as your load becomes over 100%, tap the powder. I do this by gently tapping the case head, slightly angled, with my finger over the mouth, on a piece of soft wood, then seat the bullet.

I personally don't like "over-compressed" loads. Reason being, they can cause the bullet to push back out. i have loaded some cases in my 300 RUM during load development that I need to cam the lever down several times to get the bullet to remain seated. I would never go with that type of load for hunting, but in case it was max, it's good know and shoot it. I think the best possible case fill is a very slightly compressed load, tapped out. But I shoot the max accurate load whatever it is.

Good shooting,

Mark

 

[MSLRHunter]

Thanks guys for confirming my suspicions. Fitch, thanks for the quickload data, I may need to invest in that sometime in the near future. I shot a few rounds today just slowly workiing up to those max loads, however I didn't last too long, only about 15 rounds,due to the fact that I am trying to decide if I need a muzzle brake on this rifle or not. After shooting 15 times, it was starting to hammer on me pretty good. I'll keep everyone informed on my progress with this rifle. Thanks again for everyones insight.

 

[Fitch]

Thanks guys for confirming my suspicions. Fitch, thanks for the quickload data, I may need to invest in that sometime in the near future. I shot a few rounds today just slowly workiing up to those max loads, however I didn't last too long, only about 15 rounds,due to the fact that I am trying to decide if I need a muzzle brake on this rifle or not. After shooting 15 times, it was starting to hammer on me pretty good. I'll keep everyone informed on my progress with this rifle. Thanks again for everyones insight.

Youre welcome. QuickLOAD, used with respect for it's limitations, is a good tool. It's also unique in what it does.

A 9.5lb .300WM shooting a max load for the 210 VLD is closer to 33 ft-lbs. At the bench that would be more than I'd want to deal with. OTOH: In the field, with a freezer load of meat in the scope, you probably won't feel it.

Fitch