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Old 09-06-2017, 08:03 AM
RTSS&W RTSS&W is offline
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Calculating Bore Height on M&P 10 Calculating Bore Height on M&P 10 Calculating Bore Height on M&P 10 Calculating Bore Height on M&P 10 Calculating Bore Height on M&P 10  
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Default Calculating Bore Height on M&P 10

Just bought a Night Force 2.5 x10 and a BoBro single level mount, so I was trying to figure out the bore height for my ballistic calculations, and I so far haven't found any consistent info on the web. So here is my logic of how I found the exact bore height, and if I'm wrong, someone correct me.

1)The Bobro specs state that the center of the optic is 1.420 inches above the flat top of the picatinny rail.

2) I'm assuming that the top of the picatinny gas block is even with the picatinny rail of the upper receiver. Using digital calipers, I measured from the top of the gas block rail to the bottom of the rifle barrel which gave me 1.63 inches, and I then measured the diameter of the rifle barrel which was .75 inches. So my calculation are as follows:

Bore Height = Scope height + Gas block to bottom of rifle barrel - (barrel diameter /2)

Using actual measurements I get:

Bore Height = 1.420 + 1.63 - (.750 /2)
Bore Height = 1.420 + 1.63 - .375
Bore Height = 1.420 + 1.255
Bore Height = 2.675 inches

I know some would just round off the values to 2.7 inches and I'm find with that also because I'll never shoot better than the difference between the bore height of 2.675 and 2.7 inches. The difference out to 800 yards would only change by .02 MOA which is way beyond my skill and the accuracy of the rifle and ammo that I will be using.

So does this calculation look correct?
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Old 09-07-2017, 06:58 PM
WR Moore WR Moore is offline
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The height of the gas block and receiver rails may, or may not, be equal. Depends upon the gas block. If it is, and your barrel is straight, your math looks decent. That said, knowing the exact height is only really important if you're trying to figure corrections from standard drop tables that are generally figured for 1.5 scope to bore height.

You're correct that the minor difference you observe is essentially meaningless. The nut behind the trigger is generally the greatest variable in accuracy. Besides which, the paper drop tables give you a clue as to probable ballistic trajectories. The only way you know for sure what your drops and corrections are is to actually shoot at the ranges in question. If you're really going for maximum accuracy, you need to keep a shot log and chart any differences in drop due to altitude, temperature and/or humidity.

You'll also see variations in drop due to variations in velocity. Only really a concern way out there, but it can get real interesting if your ammo has a generous velocity standard deviation.

Last edited by WR Moore; 09-07-2017 at 07:01 PM.
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