A heat gun is pointless. Putting it in a bonfire works, but if it has fibre packing, that might get destroyed. Make sure it doesn't get too hot, as it's possible to melt thin sheet steel in a hot bonfire.

I use charcoal BBQ because bonfire in my neck of the woods is both insane and illegal. I once used a gas leaf blower to accelerate the process but ended up melting my dt400e silencer so now I just take whatever comes apart apart and lay on hot coals and go take a nap. 

I use our portable fire pit and put it on top until it quits smoking it doesn’t get to hot hey it’s a good mosquito sprayer

You first need to determine the exhaust pipe is plugged up. Rarely are they so air flow restricted that a burn out in necessary. I would chemical dip a tank long before ever considering fire.

asco

I put a heat gun in mine after painting it to help it cure, and it seemed to burn out quite a bit of oil. I left it in a few hours until the smoking stopped. Judging from the air flow, it didn't seem very restricted at all.

2T bikes, which are 50+ years old, very often have clogged exhausts. BBQ sounds a very good way of cleaning them out.

You can test your exhaust using a propane or Mapp gas torch and see how much it’s smoking I’ve done that before on a low mileage bike. Can’t wait till RT325 chimes in 

Back in the 80s I was riding a trials' event on a TY Mono. This entailed a flat out blast of over a mile, between the sections.

When we pulled up at the end of the blast to open a gate, there was a really large amount of foul smelling smoke coming out the TY exhaust, when the motor wasn't running!

The exhaust was on fire inside, luckily it went out quite quick after the motor was stopped. After that, the bike ran loads better, and I got second place in the event.

Most of it will simply fall out, tap the outside of the exhaust with a hammer and see if more comes out. You have to be careful the pipe doesn't melt. Attach something to it, so it can be removed from the heat if it's getting too hot.

No danger of melting a Yamaha steel pipe on a standard campfire or barbeque.  Steel melts at over 2000 degrees Farenheit.  Red hot range is 1200 to 1500 degrees or so...  well below melting temperature.  

The same cannot be said for aluminum silencers.  They can melt if left in a fire. 

And you'll destroy the fiberglass lining in a fire...  I prefer to just burn out the silencer parts and maybe the head pipe if it's really bad.