Here are pictures of my 115VAC HF welder doing it's thing. Kept the unit on for 3 seconds per weld and they all came out good. Those lower rails are welded! As an added note the red things in the pictures are magnetic angle holders (large model, $7.49 ea.) for welding. They worked great, kept the receiver at the correct angle and height so I didn't have to use a mish-mash of crap to get the same results.
The only problem I encountered was on my last spotweld the lower rivet (modified tip) fell out. I used low temp plumbing solder to hold it in place and I didn't wait (as told) for every thing to cool between spot welds. I just kept doing weld after weld. So on the next one I will use high temp or silver solder to hold the lower tip in place.
ADVICE: #1, Practice on scap metal to get the tong tips seasoned. I used two pieces of .050" 4130 to simulate the receiver and rail. I know that the rails and receiver measure differently than .100" but it's the closest thing to it.
#2, Make the tong tip size larger, the smaller the contact size the easier chance for burn through. I made my tips around .240".
#3, Make sure you have good compression of the metals when the tongs are placed together. You don't want any looseness.
#4, WEAR PROTECTIVE EYE PROTECTION, JUST DO IT!!!!
Hope that this info helps.