7mmRUM & Retumbo

[speedbump]

I was able to do a modified 'Audette ladder test' the other day with Retumbo in a 7mm Remington Ultra Mag. The powder itself is somewhat finer than many boomer powders, so it should meter fairly well. It did take 1 1/2 months to arrive on backorder, but that's life.

WEATHER:

Temperature: 62*F
Wind: 10-20 from 5:00 to shooter
Humidity: 98% with light rain
Sky: Overcast

RIFLE:

Action: Remington LA
Barrel: Kreiger 30" 1:9" twist heavy Palma taper, no muzzle brake
Stock: McMillain Prone w/3 way buttplate
Trigger: Jewell set @ 1 lb.
Scope: Nightforce 12-42x56mm,set on 40X
Bases: Champion's Choice 30MOA
Rings: Badger Ordnance 30mm

AMMUNITION:

Brass: Remington 300RUM necked down to 7mm & fireformed, prepped but unweighed
Primers: FC215Ms
Bullets: Cauterucio 176gr VLDs seated .005" into the lands

LOADS:

86.5 Starting load - fouler
87.0
87.5
88.0
88.5
89.0
89.5
90.0
90.5 Stiff recoil, primer cratered, minor stamping
91.0 Stiff recoil, primer flattened & badly cratered, heavy stamping & bolt lift - TIME TO QUIT & BACK DOWN!!!

All shooting was done at 200 yards. The total size of the group (excluding the fouler was 2.049." The three rounds at 87.0, 87.5, & 88.0 grouped at .570." The three from 89.0, 89.5, & 90.0 grouped at .984."

Recoil & muzzle blast was noticably heavier than max loads of 5010 that I'd used last year with good results. Obviously, the next phase of testing will involve groups at longer distances with powder charges of 87.5gr & 89.5gr, in addition to a chronograph to check velocity.

All in all, inconclusive, but promising... This rifle is primarlty used in conventional LR competition, but will occasionally be used in LR coyote & deer plinking. The 5010 pulldown standard goes out at 3,360 & ES & SDs in the low teens. Average 500 yards groups for it with 5010 were @ 3.25" last year. At $4/lb, the 5010 is tough to beat for that performance, as opposed to $18/lb for Retumbo.

Your thoughts & comments???

 

[Guest]

With that heavy a bullet, I would look hard at 50 bmg, if high velocity, throat and case life were important

 

[speedbump]

S1,

I tried 50BMG last year, & unfortunately it did poorly. Some of my buds have had good luck with it in different species of RUMs, but this 7RUM hated it. There's not much 7mmRUM data out there for ANY powders.

Others I've tried are: RE22, RE25, WC860, MRP-2, IMR7828, & H1000. The 7828 wasn't bad, but only the 5010 did well enough to go the distance. I tried Caut 156s & 176s, JLK 180s, & Sierra 130s for kicks.

BTW, who does your danzac coating for you?

 

[Guest]

I do my own coating. Have you tried AA8700?

 

[Tim Behle]

Just curious, how does the 7 RUM differ from the 7 Rem. Mag.?

How many grains of water will the RUM case hold?

 

[4mesh063]

My comment is...

I'm surprised that you find the retumbo to be a powder of choice in that gun. I'd say H4831, H414, N160, W760, IMR-4831 would be better choices. How in the devil are you getting that much powder in the case? I would have guessed that 65-70gr of say, 4831 would be full or compressed, is retumbo that much more dense?

Shoot the gun at night and see how much flame exits the muzzle. If it's a foot or more, I'd choose a faster powder and less of it.

 

[BountyHunter]

Speedbump

200 yards is the absolute low end to try and do a ladder test. You can really tell the differences when you go to around 300-400. If you do it late in evening or early morning (no wind condition) it really makes the stringing and grouping definitive.

If you have the distance try it and see the difference.

Good Luck

BH

 

[speedbump]

Thanks for the replies.

S1,
I haven't tried AA8700, but it's a thought. Anyone have any results from it in a RUM sized case?

Tim,
These will hold @ 103gr of water if memory serves. This is the full sized RUM necked down to .284, not a short action magnum.

4mesh,
This is nowhere near full. It looks like there is @ 3/16" left unfilled below the shoulder. That's been a consistent problem too, most powders I've tried leave a disturbingly large amount of case unfilled. The BMG powders are better in that regard, but with this overbore, a 7RUM acts like it likes something between say, H50BMG & IMR7828. It doesn't know what it really is yet....that's why I was curious about the Retumbo's burn rate properties.

BH,
Good point. 200 is all I have access to on a day-in-day-out basis. There's a severe shortage of LR ranges available here. I have to drive @ 6+ hours north to near Iowa, or 4.5 hours southeast to Fatboy country

in Tennessee to get a 1,000 yard range. Some pals in Indiana have access to 1K, but only if the crops are out & the weather's dry. It's a tough combination to achieve around here! I can go out to 600 in the next couple of weeks I think.

Feel free to fire any more thoughts & opinions out - I'm wide open....

 

[Warren Barrett]

S1 and Speedbump, I can`t say this will help you but, these three powders are nearly maxed out in my rifle in the following:
1) RL25 - 78grs.
2) MRP2 - 83grs.
3) Retumbo - 88grs.

 

[4mesh063]

Sorry, I thought this was the 7mm RSAUM insteaad of the RUM. Yea, the Retumbo will fill to about 115 grains if I remember right. Several of the guys at the range shot it last year. Boyd shot it in his heavy gun, He could give you info about where it stands compared to H1K R25/22 and so on.

 

[Boyd Heaton]

I found with the lot # I had was.Retumbo was 3gr slower....I also found it to be more consistant in e.s and s.d's.I got e.s. below 10fps with retumbo.And only 15fps or so with the Rl25

 

[Boyd Heaton]

Phil,Killer is shooting SWEET

 

[Guest]

Speedbump

I read somewhere the other day that MRP II was slower than RL-25. The person wrote that it was looking about 5 grains slower, but there was also a comment about it being
"close in burn rate". This confused me because I do not consider a 6 percent difference close at all, and it made me wonder if there was a typo and the guy meant 5 tenths of a grain slower.

My point being that there is a big GAP in powder burn rates, right where the Ultra MAGs and Wolfs and other wildcats want to operate. 50 BMG is a tad too slow, and H1000 is a tad too fast. Retumbo is only 3 grains slower than RL-25 and does not quite equal the velocity to pressure ratio of RL-25. I think if MRP II is actually 5 whole grains slower than RL-25 it would be closer to IDEAL than anything I have tested, it would just about split the gap in burn rate, and would be very interesting to me if I was testing for a 7mm UM.

 

[speedbump]

The MRP-2 came highly recommended from a good source, but this thing doesn't like it at all.

S1, your results copy what I inferred from testing all those powders - it falls right smack dab on the cutoff between moderately slow & really slow burn rates. There was just about ZERO data out there last year when I began the project, so all the maxs I came up with were developed in-house, by necessity.

Warren B., the ones with lower charge weight at max were faster powders (for the most part), at least in mine. They left as much as 5/16" of the case unfilled below the shoulder at max pressure. About the only thing that really filled it up was H50BMG. The 5010 isn't bad, but the super fine-grained WC860 BMG powder is shallow too.

My bad on the water weight - it's 111-113gr in an unimproved 7RUM case. I checked. Duh!?!