Ok, guys I have another of my "why would you want to do that" questions for y'all.

I have a 308 Bushmanster  ORC that I want to convert to an iron sight gun.  In normal times, I might just sell it and buy the gun I want, but I haven't seen the exact configuration I want for sale anywhere, even before the panic.  So I think my best option is to change what I have to what I want.

 My first and most pressing question is what to do about a free float tube.  When I search on 308 free float tubes for Bushmaster I get some conflicting information on what will fit, and whether a standard AR tube will fit or if I need a special adapter.  My gun looks exactly like this, except I have MOE hand guards on it.  Since I was able to replace the hand guards with standard AR furniture, it stands to reason the mounting for free float tubes might also be the same.  If I understand the difference, I only have to worry about the odd size free float tube/adapter if my rifle came with a FF tube from the factory, which mine did not.



What I think I want is to run a free float out to the brake, and use a standard AR fixed Rear sight.  Where I am on the fence is with the front sight.  

First option is to clamp a normal front sight [or a folding one] to the barrel, butted up against the back of the brake, and stop the free float tube just before it.  I want to clamp because I don't want to mill for pins and I am not going to jack with the gas system.  My objective is to get the longest sight radius I can.

Second option is to go with a longer tube, and attach the FS to the tube.  This has the advantage of seeming simpler but since the front sight is going to take some abuse [this s going to be a 3 gun rifle] going into barricades and dump barrels I would want to be sure it was going to stay put.

It seems clamped to the barrel might be strongest and most durable, but the least elegant, and in the way if I decide to clamp on a scope for load testing.  A folding clamp on like an ARMS #41 or the YHM 9394 would solve the "in the way" part but be less durable.  The unknown [by me] is where a free float mounted front sight fits in.

If it makes a difference I am considering an AP Carbon fiber tube to save some weight.

Doing the front sight is simple drive out the pins after taking off the break and then the roll pin from the gas tube

No, I want to leave the gas system alone [other than removing the sling mount and the HG ring] and I want the sight radius as long as I can get, so the FS is going on the end of the barrel

Your first step is to measure the diameter of your barrel (accurately with a mic) where you want the front sight to see if anyone makes a clamp-on of that size.

Second step: everything I've read says the Bushmaster is the same as the "low" height DPMS .308. If that's true you need to be looking at free float handguards designed for these and standard AR15 types won't fit.

Tag for future info. I too am trying to find a FF hand guard in the 12 inch range. Conflicting info coming at me on what fits. I wanted it to go over the existing gas block and if need be I can mill it down. Thanks you guys for opening up this dialogue and input.
Listening to Tom Gresham gun talk radio, surfing arfcom for info. Livin large.
Thanks again op for bringing this up.

Eta; your barrel dia. At gas block is .750. Forward of that is .720

My .308 ORC is the higher profile rail, so be careful assuming this.  

Uhm, actually *my* .308 ORC is.750 for the whole thing, it seems.  What do I have to measure on the back end to see if standard AR parts will fit?

Also, my ORC has the low rail height, and came with the risers pictured above.  Apparently the original owner was a giant, because he had AR style extra tall scope rings ON TOP OF those risers...  The end result was so tall I couldn't even get a decent CHIN-weld...LOL.

The .308's use a larger barrel nut with a different thread pitch - but I don't know what the actual dimensions are. If you happen to have an AR15 barrel nut laying around you'll be able to see the difference (I did see a note somewhere that said the DPMS .308 barrel nut has a 1 7/16"-16 thread pitch if that helps).

So, I mentioned my plan to my LGS, and he says, "Come on over, we got you covered."  I said "Really?"  "Oh yeah, no problem, come over after work, and we will hook you up."  

So, over I go, after work, to be hooked up...  Disassembly went about like you would expect, with much pounding and cussing and gnashing of teeth, then was heard the utterance: "OH."  

This was not a good sound.  "Wow, that's ALOT bigger than I thought."  That was a worse sound.  So we march in from the shop to look at the available parts..."Nope, nope, nope, nope and nope."

Friends, romans and fellow wrench turners, I have completed the walk of shame, with a bag full of rifle parts and shattered dreams.


My plan not to jack with the gas-block - Gone!  


So, this has gone from a rescue operation to a salvage operation.    I have no words for the despair in my heart right now...

All is not lost! First, go to Troy's website and use their info to see if you have the high- or low-profile receiver here.

Then you can order a free float handguard from them, in either a 12" length or 13.8", or go here for some other options.

You should be able to grind off the sling swivel and bayonet lug of your existing gas block and grind it all down small enough to fit under a free float tube, or you could replace it with a low-profile gas block if you'd rather go that route.

Save some time and money. The newer Bushmasters that take Pmags. are low profile upper. Get a J/P gas block and any std. Ar15 rear Buis. Works great.



Older pictue with a magteck. I have a LMT cut down type A2 rear sight on it now.

Yep, you will need a DPMS pattern .308 specific FF handguard.  As posted above, find out if your upper is low or high profile before you order.  Personally I went with a Troy TRX 308.  Go 12" if you want a "mock dissipator" setup with the front sight attached to the barrel, or 13.5" if you want a rail mount front sight.

I agree that it is preferable to have the front sight attached to the barrel rather than the handguard, because the handguard floats independently of the barreled action.  A floating front sight is not a good thing.  It might be good enough for backup sights, but on a dedicated iron sight gun, I would definitely go with a barrel mounted front sight.  Personally, I went with the JP adjustable gas block fixed front sight, which clamps on.  It will fit your barrel fine.  They run $125 or so.

Why not install a rail and add flip sights?

But if it was me, I would not install sights. Seems pointless on an optics ready AR-15. Just mount a scope and shoot it.



If the barrel is off, you can send it to ADCO. They can reassemble it and install a FSB.

Bushy .308 ORC with Apex FF tube.



Apex  Cut off delta ring, FF tube attaches to bbl nut.

Current config...

For me when I added the J/P gas block front sight. It was becouse I like to shoot with iron sights. From time to time.