I am about to order a barrel in .257Roberts for my LR308. To do so the smith needs a bit of info on the gas port. He needs the length and size. I told him to go with rifle length, but size is a mystery to me. Should I make it a standard .308 size? What is that even? I am looking for any input as I am planning to build this next (actually order the parts) next month.
Thanks,
So you can better gauge this here is my build plan:
LR-308 Lower
collapsible stock (
Buttstock
)
24" SS Bull fluted and threaded for brake (
Barrel source
)
Muzzle Brake (
Intended Brake
)
Extended handguard (
Handguard
)
Adjustable Gas Block (
Gas Block
)
Size-wise, .257 is very close to 6.5mm. I would look at cartridges like 6.5 Grendel to get an idea of what the gas port size should be. IIRC, the Grendel gas port on a 20" barrel is around .90 inches or so.
The 257Roberts+P generates about 50k pressure, also about where I estimate my reloads are. The Grendel is the same. Good call!
With an adjustable block should I error on the large size and if so how far? .1? I am just making certain since I won't be able to afford a second go at it.
I am hoping to get this ready by May with an eye towards the VHA Jamboree. That is 20rds at 4-7" balloons out to 400+yds in a time limit.
Regardless, it ought to be a fun, almost recoil free, and one of a kind rifle to shoot. I can't wait to see it come together.
I would lean toward a gas port on the smaller side, simply because material taken away when drilling the port can't be put back. Since you're going to run an adjustable bock, why not size the port in the middle of the size range and use the block for fine adjustment? It'll also give you room to adjust downward when gas port erosion rears it's ugly head.
It just occurred to me - You might also want to re-look using a .308 buffer and spring. It might be too heavy and stiff for .257/6.5mm.
On a different tangent, why did you select the Mako buttstock and Cooley brake?
I second erring on the small side for gas port size. These rifles tend to be over gassed in the first place, so getting a head start on working up to the ideal port size is a plus.
ArmaLite Super SASS for sale W/ extras. PM for details.
The list is constantly evolving as people give me input. The buttstock will remain as is for now so the Mako is not being ordered. The hand guards are going to from JP Ent now (15.5"). The brake is still the same as I have heard nothing bad about it yet. I am hoping I can call my own shots with a gun that will likely be 14# and in a caliber that kicks about as much as a .243.
For a number on the GP size. In my constant- and mostly fruitless reading- I did find .07 thrown around a good bit. This was referencing the .243 I believe. Since my bullet target weight in 90-100gr, might it make sense to start there?
You all make good points, but I am not knowledgeable enough to use the info wisely in this case.
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