Given what you have described, there are a few possibilities. The first is that you may not actually be pushing the bullet all the way to "touching the lands with your Hornady OAL gauge. I have a couple .223s chambered with a long freebore (0.169") specifically designed to seat Berger's 90 VLD, which is a very long bullet for .223 Rem. The freebore diameter of the match reamer used to cut their chambers is very tight, only .0002" over bullet diameter. The first time I tried to determine distance to touching in the first of these rifles with the Hornady OAL gauge, I pushed the bullet in until I felt the "touch", then tightened the set screw and pulled it out for measurement. To my surprise, the bullet was seated pretty far down in the case and the measured COAL was much shorter than I thought it should be. Turns out, it was almost exactly 0.169" shorter than it should have been because the "touch" I felt was the bullet contacting the leading edge of the very tight freebore, NOT the lands. Once I figured that out, I re-measured with the Hornady OAL gauge, making sure to give a harder push with the plastic rod. Sure enough, with a little more pressure, I could feel the bullet "slide" in farther from the initial light "touch" until a true hard "touch" at the lands was reached. The tight freebore gave the false impression that the bullet had reached the lands. Although I think a steel (rather than plastic) pushrod would be an improvement, I don't view this as a design flaw with the Hornady OAL gauge, per se. Once I figured the issue out, it works just fine in my hands, although having to use extra pressure on the pushrod makes it a little more challenging to feel the second hard "touch" of the lands without jamming the bullet into the lands by 10 or 20 thousandths. It's just something you have to practice and get a feel for.
Second, I have no idea how the RPR .223 Rem chamber has been cut (i.e. how much freebore it has) as I don't own one. However, I do know it is set up to feed rounds from a magazine. For that reason, the standard COAL for loaded rounds is 2.260", unless you have modified the magazine in some way to accept longer rounds. Commonly-used .223 magazines such as a Magpul will not properly feed rounds that are much longer than 2.260" without modification. I played around with seating the 80 SMKs in one of my ARs with Magpul mags and immediately found that the 80 SMK is too long for mag-length rounds, even though it is one of the shortest ~80 gr .224" bullets available. When loaded to mag length, the bullet is seated way down in the case neck, with a significant portion of the bullet shank below the neck/shoulder junction. Although it is possible to load rounds this way, there are issues such as excess pressure and possibly having to using a faster powder to get even halfway decent velocity with the compromised [effective] case volume and relatively short barrels typically found in ARs. If you pull a factory (mag length) .223 Rem round with a 77 gr SMK bullet, which is substantially shorter than an 80 SMK, you will find that the bullet is still sunk pretty far down in the case neck. In my hands, the 77 SMK is about as long a bullet as I would choose to load to mag length. The 80 and 90 gr .224 bullets are great for bolt rifles in F-TR, where single-feeding is required, but they are simply much too long for loading to mag length without modification of the mag to accept and properly feed longer rounds. If the RPR chamber is set up specifically for mag length rounds with minimal to moderate freebore, it is entirely possible that you correctly measured the distance to touching the lands with the Hornady OAL gauge. If so, your loaded rounds are short enough that the point on the 80 SMK ogive where your caliper insert needs to seat is actually at, or below, the case neck mouth as described in the next paragraph. Your CBTO measurements suggest that you are very close to having this happen. If so, choosing a different (shorter) bullet such as the 77 SMK is your best bet. If the RPR chamber/freebore is actually cut fairly short, there is no way around being forced to seat the 80 SMKs very deep in the case without having the freebore extended, or doing it yourself with something like the PTG Uni-Throater tool. If you do that, it will be modified permanently, which may affect loading and performance with shorter bullets or commercial ammunition.
As mentioned, another possibility is that you may be seating the bullet ogive very close to (or below) the case neck mouth. In other words, you are measuring the case OAL with your caliper insert tool, rather than the CBTO, because the spot on the bullet ogive where your caliper insert should seat is actually positioned below the case mouth. Years ago, I ran into the exact same issue with Berger 80.5s, which are actually quite a bit longer in BTO measurement than the 80 SMKs. The reasons for this problem were two-fold. First, the rifle for which I was trying to work up the load had zero...repeat ZERO...freebore. As a result, I knew beforehand that the bullet shank would be sunk pretty far down below the neck when the bullets were seated at approximately .015" off the lands, even though I was NOT restricted to loading rounds to mag length. Second, I was using the Sinclair caliper insert for .22 cal bullets, which has a hole diameter just barely under the bullet bearing surface diameter (caliber). For that reason, the Sinclair caliper insert seats very close to the ogive/bearing surface junction. To solve the problem, I simply switched out my caliper inserts from the Sinclair (steel) to the Hornady (aluminum) inserts, which have a noticeably smaller hole and seat much farther out on the bullet ogive (i.e. away from the case mouth).
I suspect that you are experiencing one (or more) of these issues. It might be helpful if you can provide your COAL measurements with loaded rounds, rather than the CBTO values. Also, if you are convinced that you are pushing the 80 SMKs all the way to "touching" the lands with your Hornady OAL gauge, after tightening the set screw, hold up a naked 80 SMK next to the one in the gauge so you can see for yourself how far down in the case the boattail/bearing surface junction is situated in relation to the neck shoulder. If it's situated well below as I suspect, AND you're certain it was "touching" the lands, the easiest solution is simply to use a shorter (lighter weight) bullet.
