Mobuck,
Thanks for your input! I have frequently read that, and look forward to exploring longer range subsonic ammo at 75-100yds in the next month or two. I've been using the B-Square slip-on Mark I-III mounts for testing due to their convenience and because they still permit use of irons with only a small increase in overall height of the scope over the bore. They also make it easy to fit anything from 2x pistol scope up to 18X or 24X rifle scopes for increased precision off the bench.
It's fascinating that your best long range shooter is a 4 inches. Is it one of the heavy barrel Lipseys' models, or another variant?
After limited testing, my most accurate Mark II is one of my three KMK-10's (a final <200 factory drilled and tapped w/circled 'B' matching case & accessories). I'm hoping at least one or more of my other Mark II's will prove at least as good so I can pass it on to a true collector. I'm a shooter, and feel a little bad shooting such a nice example, but not bad enough to stop...I just work hard not to nick it up. (I also keep being tempted by Volquartsen's $400 10" threaded Mark IV upper. (Once the $200 SBR tax is gone in January, I may SBR one of those with a pistol stock, though I admit it's pretty much reinventing an SBR 10/22, which would have much better ergonomics and trek-carrying capability than a stocked pistol.)
I have a 6" Acculite barrel for my model 4938 Charger 8", but I'll have to drop it into a different stock due to it being a .920 that won't fit into that model's tapered barrel channel. That short a barrel may require me to use a 3oz Volquartsen Firefly 2.0 bolt to get reliable function; I understand the shorter barrels may not work with the factory 7oz bolt, particularly with subsonic or SV ammo.
I just picked up a near mint KMK-678 on GB for a very reasonable price with only a single magazine. It's showing great promise after one range outing. I bought it because it was identical to my first Mark II, bought in the mid-1980's, which I sold after I bought a KMK-512 to have threaded for suppressor use about three years later.
Nice as the 5.5 inchers are, I've always found the heavy taper 6 7/8" stainless and blued models supremely elegant by comparison. It's nice to have that model again. I've always liked this photo of Bill Ruger in his prime shooting a T-678:
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Here's the uncropped original, with more T-678's and his spotter partly in view:
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(The pistol lying on the table looks like it's in the white and unblued. Either that or it's blued and the bright outdoor lighting makes it look otherwise, which sometimes happens. The stainless Mark II's didn't arrive until the mid-80's, and by then Bill Ruger (1916-2002) was in his 70's and always pictured with a mustache and/or beard.)