A while ago I was working on an accessory prop to go along with the rest of my Colonial Marine gear; as I had a surplus 7.62mm (.30 cal) ammo can kicking around I decided to create a paint stencil for it and the Pulse Rifle ammo. The problem was figuring out how many rounds it could hold. I have a copy of the CM Tech Manual, but it didn't specify the 'actual' size of the round other than it's 10X24mm caseless (which we already knew.) So I looked up specs on real caseless ammo and looked at images of some of the replica ammo that's been done for the PR (such as Matsuo's) and came up with some of my own numbers. I calculated given a round dimension of 12mm x 12mm x 27mm, one could fit 990 rounds into the ammo can. (For you anal retentive types, here's the math: the interior dimensions of the ammo can are 6.75" x 10" x 3.5" and that's from online specs; my own measurements are slightly smaller but are within .125 (1/8) of an inch - my own measurement of the internal height was 6.625", so not off by too far. So doing some conversion, that's 171.45mm x 254mm x 88.9mm = 3871443.87 mm^3. Divide that by 3888 mm^3 (12mm x 12mm x 27mm) and it comes out to 995.742.)
Then I just recently received Derrick Baena's M41-A manual and his specs of the PR ammo are 14m x 14mm x 30mm. Do the math on that and it takes the amount of rounds that could fit in the ammo can down to 658; quite a bit of difference! Of course this is all speculation (and of course fantasy), but still intriguing (at least to me.)
Which lead me to make some measurements as to the PR magazine itself. As depicted in many images for the Spulse, HCG and Matsuo PR's, the magazine is quite a bit smaller than the well on the PR that it is inserted into. The magazine looks to be around 1.5" x 1" X 7" (and I'm speculating that given that I'm measuring off of an AirSoft PR mag, but I think I'm being fairly generous.) You do the math on this one, and you'll find that the magazine could only hold about 30 rounds using Derrick's ammo specs. Even given the original specs I was using for the M309 rounds, it could only fit about 44 rounds.
Measuring the mag well on my AirSoft PR, I came up with the max dimensions of 2 5/8" x 1 3/4" x 7" - and doing the math on that I calculated that if the well could accommodate a magazine of close dimensions, the max amount of rounds that could be fitted into it (given a round size of Derrick's 14mm x14mm X 30mm) would be 89. Pretty damn close to 95 rounds (or 99 if you think we could stuff that many in there; which is of course pipe dream, but hey - you gave it a try) but no cigar!
-scoff