I have this:



And am trying to install this front sight:



But if you look at the rifle (S&W M&P 15 PC), the bracket on which the sight is bolted is so close to the handguard that the sight won't fold all the way down. I moved the bracket forward enough to accept the sight, but the bracket is attached to pin that protrudes into the handguard. Is it a problem if the pin isn't seated completely into its original position? Or is the pin just to allow the sight to align?

Since unlike the OR model, your sight rail seems below the level of the main rail with the rear sight, you need the Troy front sight made for COMPACT OR LOW PROFILE gas blocks.

It will raise the pivot point, allowing you to fold completely and also properly align with the rear sight.

Troy BattleSight Front Gas Block Mounting M4 - Black

The gas block on M&P15 Optics Ready models is "same plane" as the rear sight. Yours looks to be lower. The gas block specific is made for lower height (than flattop rail) gas block rails. Hope that helps you. Can you exchange it?

Note the taller base...........................................................

No, I can't exchange it. I bought it off a forum member.

But since I've already moved the gas block forward, and then moved it back to its original position, do I need to pull the handguard and make sure the gas tube is properly indexed/aligned?

Did you try reversing it so it will fold towards the end of the barrel?

It's the wrong height sight. The mounting height of the gas block is lower than the rear rail. I've got to get a gas block riser.

Wouldn't solve the problem, still the wrong height/too short. Make sighting it in difficult if not impossible.

Since you have a piston model, I am not familiar with your set up and suggest you check your manual for servicing the gas system to make sure you haven't upset the connection or adjustment. Shouldn't be too big a deal and is pretty well described in the manual for making sure it is correct. You do however need a properly sized front sight. Maybe you can sell the one you have and buy a correct one.

How about a gas block riser?

I haven't heard of a riser but it might exist I guess. Just make sure it is solid or it will defeat the whole idea of having the high quality, rock solid troy sights.

Ooops, found this................http://vtsupply.com/yhm-gas-block-riser-ar15.html

I think I'd still give selling it or finding someone to trade in the opposite position (happens both ways too) might be the best option. Might try on ar15.com's sell/trade section.