Re: Bushmaster=hate?
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: usmc1488</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 997/2man</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Sterling Shooter</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Snake Plissken</div><div class="ubbcode-body">You guys know F marked A frame FSBs are only for carbine length setups to compensate for the shorter sight radius, right? </div></div>
Here's the scoop on the effect of sight radius: The same detachable carry handle BDC is used on both the M4 and A4. On an A4, elevation wheel and windage drum have a 1/2 MOA value per click. On an M4, the windage drum and elevation wheel have a 3/4 MOA value per click. The click value for the F or A2 front sight on an A4 or A2 is about 1.2 MOA. The click value for the F or A2 front sight on an M4 is about 1.75 MOA.
The reason flattop M4's and A4's use the taller F marked front sight base is to support a 600 meter requirement for the BDC detachable carry handle rear sight, which had to be made taller than the fixed carry handle sight to meet the requirement, since its shaft does not reach below the upper receiver's surface as does the fixed carry handle's sight stem.
There are some A4's out there assembled by RRA, DPMS, and Bushmaster which use the A2 front sight base as do A2 fixed carry handle rifles. But, when mil-spec BDC's are attached to these flattop guns they will usually shoot about 10 minutes of angle high, that's 20 inches high at 200 yards. Thing is, the folks who are putting these guns together apparently don't know they have a problem. Some folks just don't see how 40 thousandths of an inch could be meaningful.
One more thing, when zeroing the A4 for 300 meters at 25 meters, the 6/3 index on the elevation wheel is temporally abandoned with 2 clicks up to the Z mark. After zeroing at 25 meters the sight is adjusted counter clockwise back to the 6/3 index and the line of sight will now intersect bullet path at 32 and 300 meters.
</div></div>
Good post, I'm keeping this one. Thanks for the info.
</div></div>
I agree, so Im guessing that my rifle which was originaly a post ban with an A2 upper, which as been converted to a flat top, but reatins its original front site will not be accurate past the 100 zero that I did at 33. Good thing Im using an ACOG like I was issued.</div></div>
It's not about accuracy, just an issue when zeroing when a BDC function is sought from a mil-spec BDC sight. Initially, you can expect your rifle to shoot about 10 MOA high with the BDC bottomed out. This will require at least 2 full revs of the front sight counter clockwise, which puts the rifle out of spec. That's to say two full revs up and the front sight is very close to falling out of its base. The solution is an extended front sight post. I wish folks like DPMS, WW, and Bushmaster would just fit their flattop offerings with the correct F marked front sight base in the first place. Thing is folks don't complain because they rarely attempt to zero their irons.