Home Forums Hunting & Shooting Long Range Hunting 7mmx300WinMag

I remember some here having rifles of this chambering and it makes some sense to me when considering a 2.5" H&H case vs. a full-length version of the same. A 3.6" mag box should grant some throating/bullet flexibility as compared to the STW and RUM cases while giving a bump in velocity over the standard 7mmRM. My question is how much of a bump? Could a guy expect to make 3,300fps with 140's and a 26" barrel? I know I made 3,250 with a 24" 7RM, but I was standing on it pretty hard. I'd think 3,150 might be doable with 160-162gr stuff, as I did 3,250 with 165's in a 26" 300WinMag. Several here are fans of the Mashburn, but I don't know how it differs in capacity from a simple 7x300WinMag neck-down.

Just kicking around ideas, as it's that time of year...... (grin)

There's .015 difference in neck length between the Mashburn & 300 Win Mag with the 300 Mag being shorter. That difference may be made up when squeezing the neck down from the 300 Win Mag.

Jpro: The Mashburn has a longer neck than the 300WM necked down. I have never had a rifle so chambered but have necked a 300 WM down and seated bullets a few times.

I doubt there is really a great deal of difference in velocity between the two, but the longer neck of the Mashburn is helpful in arranging bullet/land relationship, especially with the shorter, lighter bullets.

Also, the Mashburn has slightly less taper than the 300WM,and is slightly blown out in forming.

I specified a H&H box for my rifle,and seat the 160 NPT even with base of neck.The COAL of the 160 AB is a bit over 3.5 and functions smoothly.

I wanted the rifle primarily for heavy 7mm bullets so I have not tried 140's in the rifle as there is little need. With Retumbo and H1000 it is easy to get around 3200-3250 with the 160,and a hair under 3100 with the 175.

The 7mm Dakota is a similar cartridge,and I easily achieved 3350-3375 with the 140 in a 24" Krieger.

I could have built to either case, but liked the Mashburn better.Had good input from Dober here and it is surprising how I easily achieved the vels that Warren Page and Bob Hagel spoke of.And it does out run the 7RM due to slightly greater capacity and COAL.Just enough more capacity than the 7RM to get the velocity without leaning too hard,especially with the heavy bullets which are very useful in this case..

When you can start a good 160 at 3200, or a 175 at close to 3100, you don't really need anything lighter.

Art Mashburn did his homework on this cartridge.

I always thought a 7mm on a .300 Win Mag case made a lot of sense. If your favorite 7 Mag is suffering from throat erosion and your rifle doesn't have a 3.6" magazine box, running in a reamer designed around a necked-down .300 Win Mag case would get you into fresh rifling. Also, .300 Win Mag brass is commonly available and it holds a little more powder. If I didn't already have a 7mm STW, I'd give this one a whirl.

Contact Tim_in_TN, he has built a couple of these and he will answer your questions. He is a great guy. Steve

Just built a 7mmX300 WBY- I thought this made more sense.

Take a look at the 284 Jarrett....

We were amazed that the .300 Winchester Magnum had never been reduced to 7 mm successfully. The wheels really began to turn wondering exactly what the volume of the case would be. We designed a different case taper, then used our traditional 35 degree shoulder along with a lead angle in the throat to accommodate 140 and 160 gr. bullets. We had a reamer made and proceeded to build the .284 Jarrett! After the case fire forming was completed, we did comparisons with the other big sevens to see where ours fit. We checked all of the cases by water volume and this is what we came up with: (volume to base of neck)

7mm Remington Magnum ..........73.5 gr. | 7mm Weatherby Magnum ..........78.0 gr.
..284 Jarrett ................................85.5 gr. | 7mm STW ................................98.3 gr.

It�s interesting to note where we are volume wise: 12 gr. greater than the Remington Magnum and 13 gr. less than the 7mm STW.

The next step was to shoot this thing! Of foremost importance to us was its accuracy. If it�s not accurate, the velocity doesn�t matter. The cartridge responded well to a variety of powders and without effort we were under our half minute accuracy with velocities appropriate for this case capacity. Our testing produced the following results:

140 gr. Nosler bullet.................................... 3450 to 3500 f.p.s.
..150 gr. Nosler bullet.................................... 3350 to 3375 f.p.s.
..160 gr. Nosler bullet.................................... 3250 to 3300 f.p.s.

These velocities are identical to a 7mm STW with a lot less powder. Even though all the 7mm cartridges are listed overbore, the question is how much overbore can you stand and still have a dependable cartridge? I do know the velocities shown are the absolute maximums for 7mm bullets, regardless of how much powder you use. The Mexican standoff: the bigger case won�t give you any more, and a smaller case won�t give you as much. This is the reason, for all the 7mm fans, we have to max out the 7mm bullet. It rounds out our performance cartridge line and gives us a big 7 that meets our accuracy criteria while producing predictable velocities. In an emergency, a Weatherby cartridge can be used in the .284 Jarrett, but I don�t recommend reloading them because the neck will be too short. Put a .284 Jarrett in your battery soon!

RD: The Jarret "is" the Mashburn .......Jarrett did not "invent" anything here........maybe the cases are slightly dimensionally different but the pratical effect is the same.Kudos to Jarrett....good idea but Art Mashburn invented it in the 50's and Page/Hagel took it from there.

This is like pretending the 7WSM is "new"....

They have been "successfully" necking down the 300 WM case for a few decades now....Jarrett is reinventing the wheel.The Mashburn is formed by necking down,and slightly blowing out,the 300 Win Mag case.The Mashburn neck is longer.

I can (have) hit some of those vels in the Mashburn, but they are a bit over the top..

3250 with a 160 is easy with todays powders in the Mashburn.Retumbo does it and H1000 comes close enough that there is no difference.I have gone over 3300 with the 160 and Retumbo with no ill effect but it is too much velocity from the case.I run it sanely at 3150-3200....it is,after all, a wildcat.

The 140 gr loads he lists are too hot IMHO....3350-3375 is doable in both the Mashburn and Dakota cases,but 3400-3500 brings us back to the Layne Simpson days and the STW,which is pie in the sky stuff..

What he does not show is that you will do very close to 3100 with something like a 175 Partition(519 BC and not bad for a retro bullet.This raises possibilities for those that like to shoot 180 Bugers, 162 Amax, etc.) A 175 Partition at those speeds stays with a 300 Weatherby 180 to 600 yards or so....

You can toss the Jarrett, the Mashburn,and the Dakota in the same hat.....identical as three peas in a pod.


I do agree that the case balances nicely between the other 7mm's and accuracy is very good...it easily outruns the 7RM IME without going over the top;seems to require less powder than the full length blown out H&H cases like the 7mm/300 Weatherby,and the STW.But Bob Hagel figured that stuff out 25-30 years ago, too...

Overall a really good idea by Jarrett but I LMAO that he wants to take so much credit for something done in the 1950's by Art Mashburn........kinda like him glomming on to the 280 AI;or Winchester jumping onto the Gradle Express cartridges and declaring them the "Second Coming"....ain't much new under the sun...

BobinNH

Sounds like you are the man I've been wanting to ask questions of - somebody with a Chronograph & first-hand knowledge of the 7mm Dakota.

Been looking seriously at semi-big 7mm chamberings. Very interested in the 7mm Dakota, but had reservations about it as I can't find too much in the way of velocity checks for the case other than the Hornady manual (I probably don't know where to look) and worried about the strength of the brass. In terms of case capacity, Dakota claims 95.0 gr of H2O on there website which is also what the guy told me on the phone when I called. I ordered a few cases and checked them myself. Granted these were unfired and will gain SOME when fired, but I was consistently getting 88 to 89 grains of H2O filling the cases with distilled water full up to the case mouth, flat across. I saw some guys posting that the 7mm Dakota case had more volume than the 7 STW and that it would therefore match or beat the STW's speeds, but when I pressed them, they admitted that they had never actually checked it themselves AND couldn't tell me what they were getting for speeds (don't think they checked and/or don't remember/care).

My questions are(finally, right?): Have you tried bullets in the 160-180 gr & what speeds were you getting? Barrel length (24")? Powder Charges? (specifically interested in the 162 Amax, 168&180 Berger VLD's).

Also - had always heard guys complaining about Norma brass (they make the Dakota headstamp brass) being too soft in the case heads. What are your impressions of the strength of the cases (how hard can you push them before needing to break out the super-glue to keep the primers seated)? Overall impressions of the cases?

Thanks for sharing your knowledge. Much appreciated.

-Ian

Nightowl:You made me dig up my notes on the Dakota,which is also a great cartridge.

I was running 74 gr RL22 with the 150 Swift SC II for velocities averaging 3225,with very good accuracy and low spreads for 3 shot strings,like in the 6-10 fps range per my notes.

When I went to 160 gr bullets accuracy fell off; I got stringing and erratic grouping,wider spreads.....the cartridge was begging for a slower propellant with the heavier slugs and RL 25 cured the problem. I have noticed the same thing with the two Mashburns we have here,and can't say that is true with all rifles,as Dober on here has run 7828 in his for years with good results.....but as a trend in our two barrels, once bullets hit 160 gr and up, these two cases seem happiest with RL25, Retumbo, and H1000.

I can get more velocity with the 160 gr bullets, easier, from the Mashburn than the Dakota,and I suspect this is because my Mashburn is set up for H&H-length bullet seating,while the Dakota is restricted to 30/06 OAL;not much difference but it is there.

In the Dakota 76 gr RL 22 gave me 3340-3370 or so and excellent accuracy with the 140 gr AB.

78 RL22 gave me 3550 with the 120 TTSX,very accurate as well.

The loads in the Mashburn so far have been 75-75.5 gr H1000 with the 160 Nosler Partition and and AB; velocities run 3150-3175 with the NPT,and up to 3250 with the AB.

I use a fireform load Dober on here gave me which is 65-IMR4350-139 Hornady, which does 3160 (low pressure),and is, in itself a capable hunting load for lots of stuff....keep in mind this is an easy goin load and the Mashburn is easily capable of more velocity with this bullet.

73 H1000 gives 3075 or so with the 175 NPT. I have wiggle room with all these loads and they are by no means over the top.My pals rifle seems to need one more grain of powder to hit the same speeds.He also does 3250 with the AB and 79 gr Retumbo,everything seems OK.

OTOH my rifle pushed to 3300 with the same load ;no pressure signs but I am uncomfortable with that load.

I got awayf rom the Dakota for a few reasons; good as it is, you might as well have a wildcat because both with it,and the Mashburn,you are sunk either way on a trip if ammo gets lost.

Second, brass is $2 bucks a pop for the Dakota,and I cannot easily form it from anything else.Some suggest the RUM case to form, but the extractor of the Dakota 76 is set up for the Dakota casehead;using RUM cases could result in malfunction.

OTOH the Mashburn is easily formed from (relatively) inexpensive 300 Win Mag cases, which is everywhere.Forming cases is no sweat and can even be done without a bullet FF load.

Plus, if I ever get sick of the wildcat thing, my rifle could easily be rebarreled to a long throat 7 RM....not as fast of course but still good.

I have not worked yet with any Berger bullets, but recently got some 162 Amax,which I suspect will do 3200 or so pretty easily in the Mashburn.

I have experienced the "soft brass" issue with the Dakota,and cases split near the neck/shoulder/ case body junction on occaission on the second to third firing....not a lot, but enough that it bugged me.

All the velocity figures you see,whether in the Dakota, or the two Mashburns, were taken from 24" barrels...all have been Krieger #2's, 9 twist,on all three rifles.

Like any rifle nut case who works with a wildcat, I like to think the Mashburn is the optimum 7 mag,however delusional that may be ....it works swell in an 8 pound rifle and 24" barrel and you don't have to lean hard on it to get a step up in velocity over the 7RM.In larger cases like the 7RUM and STW you get more vel, but burn more powder,and mostly these are 26" barreled rifles.In any event the Mashburn gets as much velocity as I can use for my purposes,and does not kick me into next week from a relatively light rifle.

Hope this helps.

Nightowl,

I'm currently working with a 7mm Dakota with a 24" Lilja barrel on an M-70 Classic Stainless.

Load development is far from complete as I've yet to achieve a happy combination of velocity & accuracy.

I'll be happy to share my results so far, feel free to PM.

It sounds like a good 160-162 at 3200 could be the answer to many questions.....

The 7mm Dakota or the 30 Newton necked down to 7 makes more sense than anything else out there.

Ditto with Bob. My Mashburn is a rechambered 7 Rem from Brown Precision. It turns 3269 over my chrony with 76 gr. of RL-25, a 162 gr. Interlock or 160 gr. AB, and an AOL of 3.565. I use Federal Gold Medal .300 Win cases and fire form with 20+/- gr. of pistol/shotgun powder and cream of wheat. Cases capacity, to the top of the neck, is 90 gr. of water. The barrel is a 24" Shilen in 8.5 twist. I love it to the point that I am building a lighter one, about 7.5 lb. just for fun!

Art Mashburn/Warren Page got it right the first time!!!!! The Dakato is an answer to a problem no one ever asked.

JPro, the 3300 fps figure is an easy one to attain with the 7mm/300 Win Mag. I never seen a Mashburn case either, but if body taper is different then there is custom dies to be considered. The 7mm/300WM can be done with a 300WM body die $25.00 and a set of 7 Rem Mag FL dies $35.00. I just tried this fall a load with 72 grains of N560 and the 140 Nosler Accubond and was looking at groups in the .4" out of the Krieger barrel. 3300+ fps.

I think so....makes it tough to justify much else.....

Which of these cases you get the velocity from really does not matter; I think arguing over which of them is inherently superior to the other is pretty much a waste of time because they all do essentially the same things,ie , provide just enough more capacity and hence, velocity than the 7RM,while not jumping clear into the capacity of an STW or RUM...which are fine too, if that is what you want.


SU's suggestion of the Dakota or the Newton case(another oldie present today in the 375 Ruger),and Tim's suggestion of the 300 Win Mag would be equally good,because capacities are so similar in all cases.....I do find it funny that even though the Mashburn set the standard for this capacity level in a 7 mag 60 years ago, only Dakota had the foresight to give us a factory offering equivilent....and anyone moaning loud and long about "slow" 7 Rem Mags can fix it easily with a pass of the reamer and a Mashburn, Dakota, or 375 Ruger necked down.....that'll fix it!

Keep in mind that the 300 Win Mag case did not exist when the Mashburn was cobbled together,and that is likely part of the reason it has a straighter taper to the case.Art Mashburn messed with a FL H&H case necked to 7mm(likely similar to the 7mm/300 Weatherby and STW).....but found it not as good as the Mashburn we talk about here,which is why this one is known as the Mashburn "Super".

I think Ruger missed a good bet in not giving us the 375 Ruger case necked to 7mm.If they had,I would not have a Mashburn....not much need.

In the end,there are lots of ways to have a case of this capacity and it's a shame the factories have not really given us one....if they had we would not have to be discussing wildcats at all...

Docbill: I have not run RL25 in my rifle yet,having sort of settled on those H1000 loads for the time being. My pal has run it in his with results similar to what you get from your rifle.

If you have throat erosion, a simply ream job will not fix it. Most barrels don't have enough shank to sufficiently set a barrel back. Besides you might get a couple of hundred more rounds...maybe, but it it is a waste of time and money. Rebarreling is the better solution by far. Barrels are not that expensive, but pulling, reaming and head spacing are, unless you can do it yourself. Flinch

Jeeezzz, Bob, relax. I was just trowing out a suggestion.

RD: I'm relaxed .....that's how I relax.....none of it was directed at you personally

Just kabitzin'

It's a good suggestion......

And tough on reamers......