While I like the original classic machine guns and would hate to be with out a Thompson and wish I had not passed on the $3500 BAR and M60 (years ago), the individual cost of a nice classic machine gun can limit the rate at which you acquire them, in part depending when you got them and the size of the toy budget (bellow $100 per week now days one has to wait a while to get a nice Bren, however, as one nears a toy budget of a $500+ per week, you fairly quickly past looking for shooters and tend to be upgrading a focused collection ;-). Thus for the younger shooters (i.e. someone that could not in the old days Form 1 machine guns with out there father ;-), who are not primarily into the history yet (this grows on you), are more a shooter than a collector and are still filling in the gaps for something to shoot, the systems guns tend to give a lot more shooting options for the same dollar spent, once the primary sear or receiver is purchased and thus are a nature starting point.
Historically the three common system guns were the M16 (and DIAS), HK Sears (or trigger pack Shattered ;-) and the Uzi (which I am excluding from further discussion as is in pistol rounds only). Over the last few years with the various new clones and clone refurbs of factory rifles by Jeff, Mike and others, the line up of options to run your HK sear (or trigger pack) in has grown to the point that for a shooter who does not need to match just the configurations made by HK, the HK sear and hosts have equaled and are surpassing the options for the M16. The three areas where the M16 has advantages are being narrowed each year. Now days one can now get optic rails on your HK clone to mount optics nearly as well as a M16, the cost of a nice upper for your M16 is only a little less than half an interesting clone and now days it is not that hard or expensive to get a barrel changed on the HKs (ok is slower than a M16 and does cost me a couple six packs of good beer extra ;-).
An advantage of the HK Sear and of course the DLO trigger pack, is one is not hammering on a $12K piece of aluminum which is an M16 (which I think of every few years when the 9mm upper comes out and which my Shrike kept trying to do and is now gone as it would not play nice). I became less worried about sear breaking when one was hammered / blown out the bottom of the trigger pack on a poorly running 21 years ago with bad but low cost ammo (note some of the old 21's were setup bad ... but that is another story) ... with no damage to the sear (trigger pack was toast but could have been rewelded easily enough). That is not to say that an even larger piece of forged steel receiver is not "more" better, just that pricing precludes one having a Thomson, BAR, Bren and 1919A4 at the same price as a sear and several clone hosts any more (did pass up on the BAR and M60 for four grand each back a "few" years ago, but I already said that).
Following is a few note on trying to stretch the HK clones to fit an ever wider range of areas, more or less, at least for us mortals can purchase only "available" transferable (i.e. excluding some of my favorites such as the M240 and M249).
Heavy Machine Gun:
Will start with the one class where have not seen any activity on a sear host for a water cooled 30 cal or 50 cal belt fed machine gun and maybe this is for the best (sure someone will post a home built water cooled gun ;-)
Medium Machine Gun
The HK21e on a tripod (I have not seen a good adapter and setup yet on the market for the "e" version, but they are being worked on) could be considered for an entry level medium machine gun or more accurately a "light" GPMG (if such a thing existed). I would rather hammer a 40,000 rounds through a pair of Browning (in particular posties ;-) in a weekend if only due to the lower cost of surplus barrels, parts and lower cost corrosive ammo (8mm in the old days and 7.62x54R). Of course there are various other "better", heavier MMGs or GPMGs with more history but a higher price than the HK21e, assuming you already have the sear.
Light Machine Gun
The HK11 tends to allow one to play with a classic light machine gun "like" configuration and the HK21e is light enough to actually fit into the LMG area, but with a belt. Now If it was Christmas I would prefer a nice Bren or even a BAR FND under the tree but has not happened recently, maybe should not have had fresh roasted reindeer that one Christmas morning.....
The HK23e clone put out by Mike, brings down the price of a SAW like weapon, compared to the original HK23e or the half dozen preSAWs in the pool. Plus one has the option of changing calibers on the MM23e. In comparison to the HK23e the Shrike on the M16 is lighter and an interesting addition for off hand work, however, due the large beaten zone I went with the MM23eK, plus my distrust of having the rear of my aluminum receiver bumped. The 223 M16 Light Machine upper (or the poor man's version with the HBAR flat top upper) can be countered by the HK13 or MM13K options.
The HK91 as a sear host is a first pass equal to the G3 and from my experience a nicer shooter than the M14 (long gone). While like the full auto FAL's, in particular the heavy barrel option, are nice shooters, I found the G3 with a good muzzle brake was similar to the FAL w/o a muzzle brake. No real option now days for a 308 M16 or using a DIAS. The area of supper compact MBRs (i.e. HK51 and the 51K) appear to be primarily an HK occurrence.
7.62x39 Assault Rifle
The AK47 is a classic and for a complete historical collection a must have, however, for a shooter the various drum cut PTR32 rebuilds or "scratch builds" by Jeff are doing well for friends. The 7.62x39 uppers for the M16 worked well except for the limited options for mags, thus leaving the DIAS with a 7.62x39 AR15 lower using AK mags as the "other" option. Some the shortest version of the PTR32 rebuilds or "scratch build" Dragon by Jeff look like they may be as interesting shooters as a Krink (which still has the history going for it, but this is a shooters point of view discussion).
223 Assault Rifle
The M16 is the classic and again a must have for a complete historical collection, however, an HK33 does fine and with a rail has a fair amount of optics choice, possible in a heavier package.
223 Assault Carbine.
The M16 carbine is the classic configuration for many fellow shooters, however, I find the HK 33K to be smoother firing on full (but again maybe heavier, depending on how much you have mounted on the CAR16). Once one gets much below a 9" barrel length the HK53 shoots much nicer than the various sub carbine size M16s that I have used, plus the stock goes shorter on the HK's. The reports in on the baby Dragon (53K) are interesting and start to make one consider a Dragon for an ad hoc replacement for a more modern PDW (which can not be purchased by us mortals).
45 ACP Subguns
A few 45 HK hosts have existed on and off, however, one has to have a reason to own a Thomson or two ;-) Other option could be an Uzi in 45 or a MAC10 with accessories.
The MP5 is a classic and must have for any complete collection. The only other common close bolt 9mm sub gun is the M16 in 9mm with the issues of being hard on hammer pins, mag issues (they self unload and a pain to load with out speed strips) and in general the M16 9mm is choppier than a MP5. The various open bolt subguns, such as the Uzi and Sterling, are nice additions to the MP5 but not really replacements for it. Again there is a whole world of historical 9mm subguns to choose from as one grows the collection, many of which start with MP....
9mm Compact Subguns
The MP5K-PDW makes a nice compact subgun that can fill in for a full size sub gun nicely. While we regularly shoot the MP5K-PDW at 100 to 135 yards, after a few mags with a stock MAC10, one tends to loss interest. The mini Uzi is another option, but one that was never as familiar with as the MP5K-PDW.
The MP5K is at the upper end or past the machine pistol range, with the most common sub gun that best fits being the older MAC11 in 380 and maybe a micro Uzi.
To e-mail shoot out the X's