Need technical input: rifle project based on 7LRM cartridge necked to .277

Apologies for the previous post if any should think I'm too long winded. But I hope it serves to somewhat clarify my intentions. In addition, there are no definitive plans so far as what I want to shoot with it. HELL..anything I can see, I want to be able to hit! Coyotes at 300yds, or a big honkin rock at 1200!
Truth be known, there's not too many places where I live to shoot long range. It's all trees and brush, anywhere that's open for hunting....or rolling cow pasture anywhere else. Every deer I've shot in the past 20 years has been inside 40yds, and more than half inside 15yds. But this isn't strictly a hunting rifle build.
I am lucky enough to have a local 100 yard range. That should serve for initial load development. There are also rumors of a private club with 600yds, and actual steel targets (only talked about in hushed tones, and heard in whispers! Real black hat, Secret Squirrel stuff in these parts! 😅)
But I like to travel some. This will give me a reason to seek out new horizons...so to speak...and meet new, like minded people. Looking forward to handing my rig over to some TRULY skilled individuals, and learning what I can while they ring it out for all it's worth!
One other thing. The intent is to build the cartridge for the Berger .277 170gr. EOL, because that seems to be the best currently available commercially. However, just checked my stock. I bought 200 count, 175gr. VLD from Marshal Ambrose over at Matrix Ballistics. They are not 166gr. as I quipped previously. This was, for any who are interested, way back before he turned Matrix over to Stephen Baardsen. This was a special order, as Marshal didn't have them in current production. He was a great guy to work with!
Also bought a quantity of out of production projectiles from member "azsugarbear" back in 2015. Is he still around? Another great guy! Anyway, he sold me 149 count Wildcat .277 195gr., and another 31 count Matrix 175gr. So....I've got those to play with.
Looked into the brass situation. Mulled it over some, and agree with you guys that the 375 Ruger cases are not the best begin. Seems ADG will indeed provided excellent brass in either 300 PRC, or 7LRM. Problem is, everybody is out, or running low...but they can still be had for around $1.80-$1.95 a pop! Wow...bend over and grab the ankles!
The expected higher price of specialty brass was one thing I hoped to avoid by starting with the 375 Ruger. But the majority which is available is Hornady. That, or ADG, which isn't appreciably less expensive than the already partially formed 300 PRC, or the 7 LRM. So...yeah...makes no sense to do anything but save time and aggravation by taking the path of least resistance, and buying 100-200 count of one of the two others!
Which will it be? The 7LRM could...should...be easily reformed in one shot, and done. But the 300 PRC might be had a bit cheaper...although will require more steps, more work, more tools. Open to further suggestions, and encouragement. In the mean time I'll go double check to see which is most readily available. Would like to get this brass order in tonight, and check it off the list.
 
if it is your dream to build a 277 LRM you go for it, do let the naysayers talk you out of it. I built a 270 on a shortened and improved 338 RUM case not the most practical but it kills like lightening. you want a 8 twist minimum with a 7 twist a better choice for the mono from hammer or badlands. good luck with your project let us know how it turns out
 
Forming the case from the 300 PRC brass may cause you issues when moving shoulder material to neck material which usually means pushing the shoulder back an extra .010 then neck turning and fire forming back out so you have excellent necks, the 7 LRM is plug and play.
 
Well, after looking again...seriously looking, with intent to buy some brass, I couldn't find any for immediate sale. Or at least not through normal retailers...didn't check auction sites. No 7 LRM, no 300 PRC. BUT I did discover one retailer with some RWS 6.5x68, which I understand is rather excellent quality brass.

Measurements on the 6.5x68mm cartridge are:

Case typeRimless, bottleneck
Bullet diameter6.70 mm (0.264 in)
Neck diameter7.60 mm (0.299 in)
Shoulder diameter12.18 mm (0.480 in)
Base diameter13.30 mm (0.524 in)
Rim diameter13.00 mm (0.512 in)
Rim thickness1.40 mm (0.055 in)
Case length67.50 mm (2.657 in)
Overall length86.50 mm (3.406 in)
Case capacity5.58 cm3 (86.1 gr H2O)
Rifling twist250 mm (1 in 9.84 in)
Primer typeLarge rifle magnum
Maximum pressure440 MPa (64,000 psi)

Base diameter is in line with the dimension indicated on the 7 LRM cartridge blueprint. Just saying, since I haven't measured it. Cartridge length of the 7 is 2.514", so this one is .143" longer.

But you know...all this right now is just blowing wind! The fact of the matter is, my best choice for brass is unavailable at the moment. Odds are some will turn up by the time it's needed. For right now I have the 100 count of old Hornady brass that can be put to use. It can be sorted, and massaged, and maybe out of the 100 there will be 50 worth farting with, or maybe less. But it will get me that much closer!
OK, so what's the next step? Order the Hornady bushing dies for 7 LRM? Who's going to have that? Then the reamer, right? It was mentioned that could be done with dummy dummy rounds. Truly? And what about the cartridge blueprint, with adjustments for neck reduction? Will that be necessary?
 
there was a group of guys that explored that route and while it fireformed without problems, the rim dia being .512 they were having extraction problems. the extractor would skip past the smaller rim leaving the case behind
 
there was a group of guys that explored that route and while it fireformed without problems, the rim dia being .512 they were having extraction problems. the extractor would skip past the smaller rim leaving the case behind

I thought about that. One article mentioned a "rebated rim", and it shows right there in the specs. But I was dead tired by the time I wrote that last night, and couldn't see it right in front of my face. I suppose if one wanted to go that route, and have something truly unique, a standard bolt face could be cut to fit the rim, right? Seems to be not too much trouble finding brass is available. AND, at $1.79per, it was less expensive then good 300 PRC...much less expensive then 7 LRM.
Can anyone think of another reason NOT to go this route? Just mulling it over here. Ideally I'd like to come up with something that could be repeated by others with the least amount of trouble and expense.

cd65x68schuler.jpg
 
there were some people that were using win mag brass before the 300 prc came out when 375 ruger cases weren't available. the rim and belt are already the right size and the body blows to fill the chamber. it looked a little funny but they claimed to have good success with it and cases had good case life
 
For any who are following, just updated the thread title to better reflect where this conversation is going.

On another note, found an article last night from four years ago where somebody did actually complete a 6.5 version of this project; 7 LRM Hornady brass massaged and necked down, complete rig...just because he wanted to! Accuracy was excellent with Berger 140 Hybrids! But surprisingly...or maybe not...he was only hitting about 3100fps with 66gr. of H1000 out of a 30", 8 twist barrel before pressure signs. Would have thought such a combination would be absolutely cooking! 🤔
 
there were some people that were using win mag brass...

Am I understanding correctly...the forward section of the brass case would blow out to the diameter of the rimmed base? Wouldn't that create a ring...a weak point...right at that junction? Or is the brass really that malleable?
 
I have not tried this personally only passing along info that I have seen on here over the years. they claim not to have problems. if you formed with a couple light loads before you go full power I don't see a problem. there were pics on one of the treads that showed a line just above the belt looked weird but claim not a problem
 
...the belt looked weird but claim not a problem

Well, that would solve the issue of sourcing brass, as it can usually be found cheap, and in quantity. Case length is OK. Pain in the rear with fire forming, though, and added expense. Wouldn't want to waste primers right now. Although, I see there are hydraulic forming dies available now. Wonder if they have enough OOMPF to accomplish something like this? Think I'll have to put a pin in this one and come back to it.
 
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