I can see why the players would want a anti-material rifle but consider this:
The Barrett M82 TODAY costs around 8,500 USD on the civilian market here in the U.S. That beings said the price has been dropping they used to be around 10,000 USD. Now I have no idea how much they cost the military but when one unit costs that much they pretty cautious about handing them out.
But why have a semi auto AM when you could have a Full-auto HMG like the M2HB?? That can really give soft skinned vehicles a bad hair day.
I totally understand not letting the PC's start out with weapons like this but why not try to regulate the scarcity of the ammo by it's quantity? I have always controlled the amount of stuff the PC's find, not really it's cost. I have never let really "buy" equipment at the beginning of the game...maybe barter for it in game and that is if they can find it. You are the GM after all you control everything down to if they feel like using the latrine or not. You can control any weapons or equipment they obtain!!!
One of the most fun games Twilight 2000 games I ever ran. The PC's were super exited because they finally ran into another small NATO unit. Who turned out to be Alabama National guard!!!! They all had M16A1's no machine guns. One of the National gaurd soldiers had an old M14. He was down to 20 rounds of ammo for it. And he only had 6 magazines for it. He was more than happy to take an M16A2,12-30 round magazines and full compliment of ammunition in it's stead. The PC's now had a 7.62NATO battle rifle and could accurately shoot out to 800 Meters without drawing as much attention as the M60 machine gun did.
The other non standard weapons the party had was an M-79 grenade launcher but they never had very many 40mm grenades for the M-79 until they found a case of them inside a collapsed bunker. They later obtained a M16A1 with a M203. They also had a civilian 12 gauge shotgun. The shotgun came in handy a few times especially when foraging but they always seemed to be Short of shot shells for it.
In another game the PC's (4 PC's all civilians except one) found a bunch of WW2 soviet weapons in a Russian museum 1
P-28 Machine gun 1, M91/30 rifle 1: SVT-40 PU Sniper rifle 1:PPSH SMG, 2:TT-33 pistols. Both the Machine gunner and the fellow armed with the M91/30 were given pistols as back ups. They later ran into a young man who joined the party and was a hunter and he had an SKS. They were doing pretty well fighting until they ran into Czech "troops" marauders who didn't kill them but forced them to trade the SKS for a CZ-24 SMG but they had no interest in there older 7.62x54 weapons. They figured since the PC's already had 7.62X25mm SMG and pistols than they had no need for a 7.62x39mm SKS. So they forced them to take the SMG off there hands.
eventually the PC's traded the DP-28 to a cantonment who needed another machine gun, at the same time they traded the 91/30. They received 3 AKM's magazines and ammunition. But they never gave up the SVT-40 (the hunter kid took this rifle and carried the CZ-24 as his close range gun) They also kept the PPSH and the two Tokarev pistols. Eventually the PPSH got damaged in battle by an exploding hand grenade and it got traded to a local gunsmith for more ammunition. The two who had the Tokarev pistols carried them with pride and they actually got them out of few tight spots.
Just keep in mind the PC's are in your capable hands.
Brother in Arms