BUIS minimum mounting distance??


September 2, 2009, 01:15 PM
I have a busmaster O.R.C. (Optics Ready Carbine) that has a flat-top all ready to go for scopes, BUIS, reflex sights, etc. to just be clamped on.

I recently purchased a set of Troy Industries flip up iron sights for it that I will use as my primary sights.
The question I have is.......Is there a minimum distance that a classic style rear aperature (selectable just like the A-2 rear sight, windage, etc.) should be from the front sight post (classic style "y" with height adjustable) to get the best/required sight picture? (I seem to remember reading this somewhere)??

The problem is that the rear sight is mounted just over the charging handle (at the rear of the flat top rail) and the front is mounted at the front of the upper reciever (NOT on the gas block/no adapter/16 in. bbl.) so there is only 8-10 in. to work with, without going to a new gas block with rail or traditional front sight. (but it appears that an after-market railed hand guard/gas block may be the only solution)

I'm no expert, but it seems that the closer the two sights are together then the windage/elevation measurements may be exaggerated. I'm assuming it would be fine for CQB type applications where you are only measuring in tens of feet vs. multiple yards....but please enlighten me.

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September 2, 2009, 11:27 PM
According to this: http://www.bushmaster.com/catalog_xm15_BCWVMF16M4ORC.asp

Bushmaster O.R.C. (Optics Ready Carbine)
Product Description:
This top quality Bushmaster Carbine was developed for the shooter who intends to immediately add optics (scope, red dot or holographic sight) to the rifle - as it is shipped without iron sights. Various add-on rear iron sights can be easily attached to the flat-top Upper Receiver, and Bushmaster’s BMAS Front Flip-up Sight for V Match Rifles (Part # YHM-9360K) can be mounted over the Milled Gas Block. The premium 16” M4 Profile Barrel is chrome lined in both Bore and Chamber to provide Bushmaster accuracy, durability and maintenance ease.

You need this: http://www.bushmaster.com/products.asp?cat=19&zoom_highlight=YHM%2D9360K
B.M.A.S. Front Flip Sight for V Match Rifles and Carbines
A high quality front sight that clamps around the V Match milled sight base/gas block - designed to be used with any open iron rear sight. This patent pending Front Flip Sight is machined from aircraft aluminum with an anodized finish, installs with an Allen wrench, locks up or down with a push button and is elevation adjustable. A standard front sight post is included, and the base is drilled and tapped for our B.M.A.S. Sling Swivel Stud (see YHM-9364 below).

Normally, the sight radius on an M4-style carbine is 14.5", and 19.75" on a full-length M16/AR15. If you mount your Troy front sight at the front of the receiver instead of out on the gas block, the front sight post will appear wider than it should, which will cause problems in aiming at distant targets(far enough out, say 300-400 yds, and the post will appear wider than the target itself, defeating your chances of holding a proper, consistent sight picture/point-of-aim). Also, if you use a different rear sight other than your Troy rear sight that has a Bullet-Drop-Cam, the BDC settings are calculated with a full-length sight radius; shortening your sight radius by 7-8 inches will force you to re-calculate your come-up clicks on your BDC yourself(most probably by trial-and-error shooting at different distances) and re-mark the cam drum, or stick a label on your stock with the come-up clicks for the different distances.

Also, with both sights mounted on the upper receiver, there's precious little space for an optical sight like an Aimpoint or Eotech, unless you get one of the newest compact models like the Aimpoint H1/T1 or Eotech XPS.

IMO, the "ORC" is meant to be a competition rifle(USPSA 3-gun, etc.) with some sort of optic as its primary sight; running one with BUIS only as a patrol/CQB/entry rifle seems like a poor way to save $80-100 over an iron-sight-equipped model like the "Patrolman's Carbine." Now, you either have to spend another $106.50 on the BMAS front sight, or $56.50 + installation on a new gas block so you can use your Troy front sight...or, settle for a carbine that has about the same sight radius, and practical accuracy, as a long-barreled revolver or a long-slide pistol.

September 3, 2009, 06:38 PM
Thanks KOR, I really appreciate it.....
I did look into the Bushmater part and that's probably what I will go with. I "should have known better" with regard to the ORC but I didn't do my reading (it was a hard-to-pass-up deal at a gun show). Thanks for the measurements too....I knew it didn't feel right.
Thanks again.

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