Convinced Dad to get a police trade-in Bushy M4 (16") as a beater.

Turns out his eyes just aren't working with carbine length sight radius.  He does OK with middy and fine with rifle.

So... the cheap beater has turned into a big ol' project instead of being... well, the cheap beater.

Anyhoo, he wants the option to flip down the sights, so my plan is to shave the A2 front sight base, installed an ARMS 41-B flip down sight at the proper rifle length distance, and include a triangle cap so he can put A1 triangle handguards on.  Yes, I know it will look like a complete mutant of a gun.  You try arguing with Dad.

Any flaws to this?  Shaved FSB not gonna work with standard handguards?  Clamp-on FSB sight not gonna jive with front end?  I haven't taken calipers to it yet since the gun is two hours away.

Is the lack of "shoulder" on the barrel going to cause heartache with positioning the front sight?  It would retain the carbine length gas system, but it needs to hold handguards securely.

If I get a full-blown dissy barrel, the cost of the whole project doubles.

Anyone done an M4 to Dissy conversion before?

Why not just mount him an inexpensive red dot like the Bushnell TSR-25 or Primary Arms?  You could do the sight & riser for under $100 easy

Save the Irons for last ditch hand to hand last stand...

For using irons, you would be better served getting a LONG free float tube and mounting BUIS to it at both ends.

You could most likely buy a completed upper with that configuration and recoupe 1/2 the cost at least on the used bushmaster upper if you don't butcher it...

http://www.primaryarms.com/fu16-5-56soc-15fhr-zs

The plan is to make this his "red dot" rifle.  He has a 20" A4 (irons) and a 16" middy (scoped).  He'll have three sight options between the three guns.

He doesn't want rails or a tube.  The aesthetics of the A-frame FSB with a little retro mixed in suits him.

He even made custom A1 middys.  This gun currently has ARMS 40L and scope.

Other than the barrel being too small in diameter where you want to mount the Arms #40L, no issues.

Do you have a caliper? Measure the area in front of the m203 notch. I've converted several bear creek m4 barrels to dissapator, but they are very close to .750 where the FSB mounts. I've measured a dpms barrel and it was quite a bit smaller. You have a couple issues if it is to much smaller, for example if the barrel is .730, the difference is .020, so half of that is .010, that means your sight post will have to be that much higher. Some people have talked about cutting pieces of a feeler guage to make shims, that'd probably work if you need it. I install my fsb with the handguards so that I get the length right and I bore sight it to get it straight, but I live fire test it to verify zero before I locktight it on tight. In the past I found a roll pin that fit the gas tube hole, but it was a little big so I had to drill the hole out, but now I think it'd be better to just get a gas tube and cut it off so it sticks out about 1/2" to hold the handguard end cap. You don't want to ruin that arms sight in case you decide to repurpose it. I also peened the cap where it gos around the barrel to make it tighter.
If you need to make shims from a feeler guage the equation is: .750 - (your barrel diameter) = (the difference) ÷ 2 = (the size shim you need)

Old eyes love RDS.

I will get a measurement on the front end, and I already planned on sacrificing a gas tube.  Thanks!

You could also sell the bushy upper and buy a Dissipator upper from psa.

Arms sells an oem version of the flip up a frame that can be pinned in place.

What about this  https://www.botach.com/kley-zion-carry-handle-mount-for-aimpoint-holosun/

then just drop on a aimpoint/holosun/sig red dot

Yup, I did it and it was easy with basic tools.

1. I taped the gas tube hole on the dissy FSB and used a socket head cap screw to hold the HG cap in place. The screw head also fit nicely in the gas tube cut out in the HGs.

2. I cut a shim from a Coke can that went 3/4 around the barrel to make up the difference in diameter. 3/4 gave enough clearance for the set screws I used in the bottom of the FSB to hold it on the barrel.  If you use a clamp on FSB you're good to go also.

3. I put the HGs in place and slid the new dissy FSB on and tightened it up.  It may take a little trial and error to set the mechanical zero centered, but it's not difficult.

I've always liked the looks of the Dissy and finally decided to make one on the cheap last year. I've been shooting a G'vt A2 cut to 18" for years, so thought I would really like the slightly more compact Dissy. It's OK, but the 20" chopped to 18" is more lively and the Dissy more of a slow pig.  It's going to be disassembled soon and will be installing a modern FF HG. I use these in the woods hunting deer and aren't range only toys. The extra unneeded weight out front definitely affects balance if used away from the bench

advntrjnky

Tap the front sight base and install a screw to secure the hand guard cap

There's no reason to do that. The handguads will hold the cap forwards. And he doesn't want to ruin the fsb in case he decides to use it on something else. I'm not sure if you even need anything, I saw a pistol caliber upper recently and they just left the front handguad cap free to rotate. Think about it, the handguads keep it pushed forward and the handguads index off the barrel nut to prevent rotation.