Not sure about the floorplate. The DBM's (at least mine), I can load a bit over 2.900"

 

You are going to have to determine your max Cartridge Overall Length by determining exactly where the lands are in the chamber. Back up the max by .020in (you DO NOT want the projectile in contact with the lands) as if they are engaged, you will have a pressure spike. Projectile differences need to be taken into account so you will have to do this for every different projectile you use. Once you determine the max COL you are going to have to measure the DBM interior to get max COL for the DBM, again you need to subtract some clearance space for ease of cartridge movement in the DBM. Then you will need to do the same to get the max COL for use with a floor plate, again subtracting clearance room for cartridge clearance.

This is the only way you are going to find this out unless someone has already done this. But do not forget, all barrel lands are not the same and what someone else has measured will probably not match yours. The difference will be minor, about .002 inches or less. The ogive curvature of the projectiles also vary wildly.

If the lands are really deep, you will never get near them no matter whether you use a DBM or floor plate. This is highly unlikely though.

IIRC, your rifle is already a sub MOA shooter as it is currently set up. This might not be worth the time or investment to gain anything if that is your end game. You are going to need an AOL gauge, some modified cases for use with the AOL gauge, a Comparator and inserts, a good set of calipers, and of course the projectiles for the ogive curvature.

No two rifles are exactly chambered alike. There are always variations. They are miniscule to outrageous based on manufacture techniques, materials, and even temperature of the barrel metal when chamber reaming is done.

The published MAX COAL is based on SAAMI specs and depends if the chamber is .308 or 7.62mm. There IS a difference between the two.

 

I just did some measurements on my A5M XP, not the same as the A3 but it should give you an idea.

My max COAL for a 175gn Sierra Match is 2.830 in which the projectile touches the rifling, well below the length of the 2.900 inches of the TBM. Again, the max COAL will vary because of the ogive (curvature) of the projectile when you use the OAL gauge and comparator methods of measuring the chamber. While my reloading books list a max COAL of 2.810, this is because of the differences with the chambering of the various firearms taken into consideration and SAMMI specs for the specific chambers.

Now my instruction on the AOL gauges indicates to lose about .020 to .040 inches off of the measured max COAL to back the projectile off of the rifling to prevent the pressure buildup. Think of it this way. If you park you car against the curb and stop and then try to move forward, you have the very difficult curb bump to get over it initially, which required gunning the engine to overcome the curb bump. But if you back up about a foot then go forward, you will easily get over the curb (providing it is not 6" tall), and the same principal pertains to the projectile set back from the rifling.

So if, and this is a big IF, if your chamber of the A3 is like the A5MXP, the floor plate will do nothing for you in terms of what you are thinking because the TBM already exceeds the max COAL of the chamber. But you will have to either measure this yourself, or wait to get a measurement from someone with an identical rifle.

Good luck....

 

TNX for the reply. Just have been reading some articles on accuracy and I wanted to see if others, besides Benchrest, were doing this.

 

Some do. I think I'm loading to 2.805 for 168 and 175 SMK's and they're getting me 1/4" or better at 100 yards when I do my part. I have not figured out how close to the lands are on my rifle as I feel I really don't need to considering the accuracy I'm getting.

 

Some do. I think I'm loading to 2.805 for 168 and 175 SMK's and they're getting me 1/4" or better at 100 yards when I do my part.

Lol everyone gets 1/4" or better groups on the Internet when they do their part

Shoot this challenge & see if you truly have a 1/4" system: Sniper's Hide - Long Range Shooting, Precision Marksmanship, Gear Testing & Reviews - Scout Front Page

 

I do need to do that one of these day's, but the rifle is a sub 1/4 moa rifle (multiple shooter's using 168 FGMM have shot sub 1/4 moa groups. Factory target is 0.22" at 100 yards)

 

3 shot or 5 shot or more groups? I have a bunch of targets too that are .1s and .2s, with my best ever being a .108 5 shot group. But after running that challenge & having to shoot multiple 5 shot groups at one sitting, you'll really get a strong dose of reality and what the rifle and you are capable under actual firing conditions and not random good groups here and there. With consistent sub .25moa groups, I expect to see your name at the top of the leaderboard

 

5 shot groups. FN factory targets are 3 shot groups however.

And that's some good shooting on your part. Next time out, I'll print out one of those targets (I'm noticing the SPR's are one the top factory 308's from the looks of it in the link)