There are a number of .50 cal semi-automatic rifles that can be purchased here in the United States without having to worry about getting the special BATF automatic weapon license because they are semi-auto...but they are not cheap, and they are not light to tote around.Another crazy idea I have had for some time was a semi-auto 50.cal, like an M82, but with a massively cut down barrel (half current length), hell, maybe even a bulpump design to further reduce overall length and weight, to be used as a frontline assault weapon in urban combat.
Doesn't matter if the enemy has take cover behind that wall, I'll blast straight throw it and nail him anyways. Suicide car bomber heading towards my OP with a boot full of ordinance? Let's see him keep coming with his engine pumped full of 50.cal shells.
Even against conventional military enemies, such weapons could be hugely useful. They make body armour a joke, and would also be very effective against enemy light armour and helicopters.
Barrett Model 82A1 or M107A1 at about $8,500 or $9,500 respectively, SERBU's BFG-50A at about $6,500 (You can get their 30" single shot .50 cal for about $2,400), Cobb FA-50T at about $7,000, EDM Arms Model 05 at about $9,000, and others.
You can even buy a M2HB Ma Deuce semi-auto. The JNC Browning M2HB .50 Cal is one of these. It is belt fed and comes with a M3 Tripod. But, hehehe...it will set you back about $11,000.