Bushmaster XM-15 front sight

**Kurt_D** · March 22nd, 2012, 04:24 PM

I have a BM XM-15 E2S. It is in the A3 configuration.

I've measured the height of the front sight (barrel top to flat of the sight top).

The measurement is 1.915".

I'm somewhat confused because the numbers I expected were either 1.94" or 1.98" (F-marked - did not think this would be the case).

I'm considering removing the carry handle and adding a BUIS, co-witnessed with the Iron sights. With the FSB measurement being so much lower than either of the two expected values, I'm wondering if the available BUISs are going to be a problem on this rifle.

Anyone familiar with this condition on BMs?

**red dog** · March 22nd, 2012, 05:38 PM

front sight is adjustable for height. use front sight tool or a pin punch will work. also keep in mind a3 front sight posts are taller than a2 versions. i do not recall the height specified for zero setting but it was discussed here several months ago. try doing a search you may find that discussion. if not let me know and i will dig out my service manual and get the specifics for you.

**Kurt_D** · March 23rd, 2012, 12:08 PM

Thanks for the reply, red dog...

I am aware of the adjustment procedure for the front sight. The problem is that (from what I am given to understand) most flip-up BUISs are set up for an "F" marked FSB which should have a dimension of 1.98" from the barrel top to the sight flat area. Standard (or unmarked) FSBs MAY be 1.94" and BM has posts available that raise the top of the sight post by 0.04" to compensate so that the bottom, flat area of the post is approx flush with the flat of the FSB when zeroed.

What has me flummoxed is that the dimension on my BM is 1.915", another 0.025" lower than the non "F" type. That means I'd need a modified sight post that is 0.065" taller to keep things "kosher".

Before I go on a wild goose chase or get all bent about it, I want to figure out what the heck is going on here or if I need not be concerned about the discrepancy at all. A call to BM was of no help.

Here's a pic of the dimensions I'm talking about:

**red dog** · March 23rd, 2012, 06:54 PM

dang it, now i am out measuring my sight bases. not one of them is the same. i wonder if it is just manufacturing differences between suppliers. personally i would pop a sight on and see what you get for results. i often find myself worrying about stuff that does not make much of a difference hopefully this is the case for your front sight base. good luck, still makes you go hmmmm don,t it?

**Kurt_D** · March 28th, 2012, 10:43 AM

Originally Posted by red dog

...i often find myself worrying about stuff that does not make much of a difference hopefully this is the case for your front sight base. good luck, still makes you go hmmmm don,t it?

Oh, most definitely! The gray mush between my ears gets me into more trouble (by thinking too much) than I really want to admit.

If I figure anything out, I'll post back...