I have one and have always had problems with Federal primers because they are rounded. I purchased the firing pin and spring kit on trapshooters.com from Jim Flynn and it has helped the problem. When I took it to the local gunsmith to get the kit installed he showed me why there was a problem in the first place. The firing pin hole was drilled off center so the firing pin is not striking center. I contacted browning service since I purchased the gun new and was informed they quit servicing that firearm 2 years ago.
Factory load or reload? If it is a reload, throw a new factory shell in there and see what it does. I had a BT99 that was very fussy about primer depth on worn AA cases with WIN209 primers. I switched to FIO616 and never had another weak hit. It was all a shell problem not the shotgun.
I had a weak hammer spring on the original as it's easy to replace it. Also make certain your firing pin isn't broken; had one of the original brittle firing pin models. I had some light hits because of my fault as my MEC loader did not set the primer deep enough on some worn shells and when the hammer fell it would move the primer forward and cushion the impact appearing to be a light hit; I would turn the shell 180 degrees and it would fire. I never preferred a primer that is round such as with the shape of the older Federal primers. I use the flatter shape primers. Also, never leave your BT-100 cocked when stored. BT100dc
Went I start having problems with my BT100 like you are I replace the mainspring that works the hammer. Buy them from Browning they are cheap and easy to put in. A small vise helps to hold the detachable trigger Also if you are good with you hands take out the firing pin and its spring and clean the channel it goes through. You will not believe all the carbon that you may find in there. If you are not handy spray some lighter fluid in there. I found in the cold weather RIO and Remington Primers were the first to give me problems. If you shoot the gun hard change the mainspring once a year. Do not put oil in the channel the firing pin is in. Silicone ok.
Had the same problem with my bt100 replaced the hammer spring [ main spring] and my troubles went away. Spring was'nt broke, my guess just weak after 70,000 rounds. Very easy to replace, you'll find it easier on one side than the other left or right , sorry I don't remember which.
Just replaced the firing pin and firing pin spring from Jim Flynn, and the gun still will not fire. May I ask how difficult or easy it is to replace the hammer spring? Is there a special tool that can make it easier to do so? Thanks in advance for any help. JM
To replace the hammer spring (Mainspring) in a BT-100, the following procedure has worked for me. The only special tool you need is a very small roll-pin punch.
1. Remove the trigger assembly from the receiver.
2. Pull trigger and gently lower the hammer to the fired position.
3. The hammer spring has a rod through the center of it (Browning calls it the "Mainspring Guide"). The rear end of this rod passes through a bushing (Browning calls it the "Mainspring Guide Receiver".
4. The Mainspring Guide Receiver is held in place by a tiny vertical roll pin. Drive this pin out and everything (Mainspring, Mainspring Guide, and the Mainspring Guide Receiver) is freed.
5. Put the new spring on the guide rod. Position the nose of the guide rod in the recess on the back of the hammer.
6. Re-install the Mainspring Guide Receiver (the bushing), making sure the tail end of the guide rod enters the bushing as it did before. You'll need to push a little to compress the Mainspring in order to snap the bushing back into its hole. The toughest part is getting that little roll pin back in. (Having a spare roll pin on hand wouldn't hurt, in case you bend the original).