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Old 02-24-2018, 08:03 PM
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Taper info on BSA International MKII Heavy Barrel



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Taper info on BSA International MKII Heavy Barrel.

Picked up a MKII in pretty decent shape recently to use in ARA matches this summer. Came with 3each 3/8" scope blocks installed on barrel. I want to install a picatinny style rail from the middle block area to the breach and have it extend about 1.5" over the receiver but not touch it so it "floats" with the barrel. This particular base is made out of 7075-T6 aluminum so it is very tough and up to the task. But i need to machine it with a taper to get somewhere between 0 an 10 MOA for the scope I want to use (Mueller 40x fixed) at 50 yards.

https://www.brownells.com/optics-mou...prod44961.aspx

Trying to find the taper info on the barrel online has been fruitless (even here on R/C, looked through a lot of threads), so out came the dial calipers for measurements every 1"along the length of the 28" barrel.

Double check my math theory here for the next few paragraphs:

Breach diameter is 1.120" and muzzle is .870" for a total difference of .250" across 28". That works out to .00892" per inch mathematically. Measuring every inch confirmed that number as I got .009"/inch taper with the dial calipers.

So now that I have the taper rate for the diameter of the barrel I need to know how much angle I need to machine the base for. Since I am only machining for one side of the barrel, I divide the taper rate (.009"/inch) by 2 to get .0045" per inch for the barrel taper one side. This will give a flat 0 MOA non-taper angle for the scope rail



If I want a 10 MOA base declination then I need that value
1 MOA at 100 yards = 1.047"
1.047 divided by 100 yds = 0.01047" per yard
.01047 divided by 3 feet = .00349" per foot
.00349 divided by 12 inch = .00029" per inch for 1 MOA

So if i want a 10 MOA base I need (1 MOA .00029") x 10 = .0029" per inch machined taper, subract that from the 0 MOA value of .0045"-.0029" = .0016" per inch of taper machined into the base.


Curious as to what the MOA taper is on the stock barrel? Stock barrel taper =.0045"/inch divided by .00029 = 15.517 or roughly 15.5 MOA for you guys that just bolted a straight rail on the barrel.

For you MIL guys:
MIL value 1 MIL = 3.437 MOA
MILS = 15.517 MOA divided 3.437 = 4.514 MILS
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Old 02-25-2018, 09:46 AM
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Do it like the old timers. Shim the blocks to level.
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Old 02-25-2018, 11:46 AM
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Quote:
Originally Posted by Carney Pace View Post
Do it like the old timers. Shim the blocks to level.
"Or", try differing blocks until you get it right.
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Old 02-25-2018, 01:35 PM
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For mounting scopes on my MK II I used one of these:
https://www.brownells.com/optics-mou...prod40900.aspx

Well, actually my rifle came with one of those, so I cannot say which one. At $10 each, I would buy both and save the extra for a future rifle.

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Old 02-25-2018, 02:04 PM
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I used one of the rails from Brownell and mounted a 36X Leupold



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Old 04-01-2018, 11:20 PM
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This project turned out pretty good.

I wanted a 10 MOA rail as I wanted to keep the scope optically centered my math figured that there was around a 2.7"-3.4" drop at 50 yards for standard velocity .22 and my scope centerline is about 2.375" above bore for a total of 5" to 5.5" difference. It worked out to somewhere around 8.9 to 9.7 MOA, round up to 10.

I even cheated and put it in a CAD program and verified my numbers above. Put the rail in the mill and 1" ballmill a .024" tapering cut to .000" over 14.25" for the angle. One screw-up on my part was cutting the overhang over the receiver as I miscalculated in my head and took .1" too much off, but it will work, just didnt quite get the look I was after.

Rail bolted on in existing scope block holes at two places, totally free floating with barrel

I may redo the rail and cut the rear 3" off and then I wouldn't need the sunshade and get the receiver overhang lower, but it works for now



With the Mueller 40x installed, just have .050" clearance between scope bell and rail, not enough room to put the scope cover cap on. Installed the sunshade and it gave enough length to fit the cap without hitting the rail.

Last edited by wolfshoon; 04-02-2018 at 05:31 PM.
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Old 04-27-2018, 12:14 AM
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I'm pretty happy with this project.

ARA test targets at 50 yards, 5 rounds per bull in some pretty difficult wind conditions (variable 5-12 MPH at 7-8 oclock switching to 5-12 at 3-4 o clock, nothing consistent)



Next part of the project is to actually clean it, the barrel borescoped pretty ugly with a lot of crap in there. This might take a couple of hours.
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