Decided to throw the LMT M203 on the AR as it was intended. The M203 slides over the barrel and index on the barrel nut. It then clamps to the barrel. The KAC QD M203 mount has an insert that allows you to adjust the length of mounting area. Depending on wear and tolerance stacking you use this insert to make sure that there is no forward/backwards play in the launcher.
This spacer solution that KAC came out with is rather nifity. In their QD mount this insert is X shaped with 4 U notches in it. Each notch is cut to a different depth, allowing you to pick between 5 different thickness to take up any potential slack in the mounting.
Yes 5 options. The 4 notches are each different thicknesses, and if they are not enough, you can flip this spacer around, to use the backside, for a 5th option.
I should have taken a picture of it for this description.
Anyways, the rambling was to explain that the M203 launcher puts the barrel in tension, applying forward pressure on bottom of that M203 step cut into the barrel.
On this gun, that pressure moved the point of impact about 5 MOA up. I adjusted the zeros on the iron sights and the ACOG. I started at 25m, then adjusted the ACOG zero again at 100 yards.
Since the M203 was moved to this gun, I put on a leaf sight for it on the top rail. Fortunately it was right on, and no adjustment was needed. (Well, it impacted about 8 inches high at 100 yards. 1 elevation increment on the leaf sight would move the impact about 5 meters, so I choose to call that close enough)
Best part for me is that my stand alone mount has its’ own sights. I’ll be able to just pull the M203 right off this upper and slap it on the stand alone mount when I want to run it by it self, and I can throw it on this upper when I want to run it on an AR. I’ll just have to remember the 5 MOA point of impact shift should I take the launcher off this rifle.
I recall back when I was in the Corps, some of the M203 gunners liked having the M203 on their rifle during rifle qualification as the extra 3 pounds helped keep the rifle steady and made for less movement during firing. Personally, unless I knew I was going to be lobbing a bunch of 40mm, I’d rather have it off my gun. But here I am throwing it on a gun anyways.