Be careful about that, the M1/M1IP and M1A1 used different sensor mountings. The 120mm uses a mount machined into the top of the muzzle which the sensor bolts onto:
The 105mm has held on using a crappy clamp system (did not work very well):
The 105mm muzzle is also visibly smaller.
Ken
I also just remembered; I have to modify the left-hand side of the hull to add the fire suppression system handle and the circular hole, which I assume is an exhaust port, right?
Its for the personnel heater.
Thanks for the heads up, Ken. I can just grind the MRS from the kit muzzle and glue the MRS I have and add some .010 strip styrene and a little rod on the bottom. even better. 
Good, I was still on target. 
Over thirty years ago I used .010 strip when I scratch build my IDF 105 barrel with thermal shield. I was quite pleased with myself.
A few years later I used bare metal foil for the retaining straps on a hose inside this Merkava. I’ll never go back. It’s as close to scale thickness as you’ll ever get, and you can tone down the sheen by sanding it if you want. The foil’s color will contrast with the the barrel color nicely, .010 strip is .35 inches in scale, and just won’t look right.
HEP-fired that only a couple times.
Back on the subject of the MRS; I discovered a picture posted by @Delta21 of his M1IP and noticed his vehicle did not have an MRS.
SInce ken did note that the straps holding the MRS on those early Abrams were flimsy, it wouldn’t be outside of the realm of possibility that the MRS would’ve been “knocked off” the gun while traversing through the forest.
Just analyzing possible scenarios. I have not finally decided what color/camo scheme i want, but i am steering towards either full green REFORGER 87 “Certain Strike” or CAT-87 Competition “Bill The Cat” Abrams (Ack!).
Knocking off (or having if off) the MRS is a big deal, doesn’t matter to your project but its extremely rare.
Yeah! I can hear the Statement of Charges being typed as I posted that. 
Big ticket item right there. 
@Dan The MRS did get knocked off, or more commonly, knocked loose, a lot; that’s why they went with the solid mount on the 120mm version. Sometimes they ended up hanging half off the end of the gun, or slipped down the side. As I said, it was a real crappy system.
As for the round in that one photo, courtesy my brother in Korea, take another look at it. That’s not HEP, that’s WP. Here’s another photo:
HEP was OD:
Ken
You’re right of course about the ammo.
I’ve never seen or heard of our IPs having problems with the MRS’s.
I don’t know if he ever completed it , but “MikeyBugs” was working on a very nice looking M1/M1 IP MRS 3D model . I’ll try to dig up the post.
Edit: looks like he did MikeyBugs 3D Design and Printing Ideas - #620 by MikeyBugs
It’s looking good. Sure would be nice if somebody made the M1IP gun and mantlet as an aftermarket add-on - or even a decent OOB M1IP in the first place - but in the meantime you’re making a very nice job of it.
It is looking good. Way more work than I would want to do on an old Tamiya kit, but more power to you.
GIven the amount of extra work; I would’ve perhaps gone the way of Panda, but, I needed the practice and so far, no regrets. I see a lot of folks here doing a ton of scratchbuilding on kits, so this will give me that practice.
Here’s the thing though, the details on this almost 40 year old kit are sharper than other Abrams kits not half it’s age.
