The only surplus ammo in 54R that was reasonably worthy for accuracy was the Russian tins of 1960s surplus featuring a 182grn lead core full copper plated steel jacketed Match bullet. For cheap fun shooting it's a gas to play with.
But most surplus ammo and generic import rifles certainly will not bang steel reliability at 300 yards much less farther.
I shoot open sights quite often myself, here's a pic of a 1,000 yd 300 Win Mag,
It's a Remington 40X in a McMillan stock with Centra Sights.
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Here's a Browning BPCR rifle in 45-70 carrying Montana Vintage Arms Vernier Buffalo Sights. I shoot ( black powder cartridge rifle ) BPCR steel silhouettes with. You shoot steel animals out to 5 - 600 yards depending on range. And I shoot Long Range Gong shoots out to 1200 yds.
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I understand LR shooting quite well Sir, I have been actively shooting it for well over 40 years now.
It ain't banging a 24" piece of steel at your buddy's back yard, it's a great game. Get some high quality sights on the grendel and head over to Ft Meade or Quantico and do an Across the Course Clinic { high power } or scope it and hit the F Class matches at Bridgeville, De or Quantico.
You will sell those 54R's.. :1bdz :
.edit..
My son shoots alot of 54R, he has taken to collect WW2 rifles from across the pond, he has a few round and square 91/30's one 44 and a few 98's, lately he has been looking at what Enfield to buy first.
I constantly bust his balls about that Rimmed Cartridge also.
We reload what he shoots, years ago I bought a can of pulled silver tip .311 bullets cheap. Plus no corrosive cleanup to worry about.