I'll have to put my thinking cap on for this one, and possibly get a few photos over the next couple days, so stay tuned. The most puzzling aspect is (as you said), there is different behavior depending if the hammer is cocked or not. Random thoughts so far:

• As you suspect, it sure sounds like the Top Lever and the Top Lever Dog are not pulling the Locking Bolt far enough rearward to allow the gun to open (and we're talking fractions of an inch here). But in theory, that shouldn't depend on whether or not the hammer is cocked.
• I just tested one of my BT-99s . . . try the same test with yours:
  • a) with hammer cocked, remove forearm and barrel.
  • b) return the Top Lever to its rest position.
  • c) hold the front tip of the Cocking Lever stationary and exercise the Top Lever open and closed, noting how much effort it requires to move the Top Lever and therefore how much effort is required to move the Locking Bolt back and forth.
  • d) now with Top Lever in its closed position, pull the trigger and drop the hammer.
  • e) repeat step c . . . in other words, with the hammer now uncocked, exercise the Top Lever open and closed and again note the effort required to move the Locking Bolt back and forth. On my gun, there is no difference whether or not the hammer is cocked, and there is no difference in distance the Locking Bolt moves back and forth. To me, this is all expected since the Top Lever and Top Lever Dog really moves only the Locking Bolt . . . which means I'm therefore assuming that an uncocked hammer doesn't ever apply any pressure to the Locking Bolt in a properly functioning BT-99. If it is applying pressure, I suspect some small dimensional problem with that area of the Locking Bolt.
• There may be some small dimensional fault at the rear of the Cocking Lever . . . that's the part of the Cocking Lever that actually holds the Locking Bolt rearward and therefore latches the Top Lever open (see illustrations below). Maybe the Cocking Lever is allowing the front of the Locking Bolt to protrude from the breech face just enough to cause interference . . . and as you said, there's always a little mechanical "slop" here whereby the Top Lever can be pushed open a little additional amount to draw the locking Bolt rearward a last fraction of an inch and get the gun open. But, again, I can't figure out why this potential fault would depend on the hammer being cocked or not . . . except if when re-cocking the hammer, perhaps the Cocking Lever doesn't engage the Locking Bolt quite the same way, and doen's hold the Locking Bolt quite as far rearward. Puzzling!
• The fault could be somewhere in the entire cocking chain, starting with the cut-out in the forearm iron that engages the front tip of the Cocking Lever . . . to the Cocking Lever itself, (especially the critical rear portion of the Cocking Lever as already mentioned), to the Locking Bolt surfaces (might need a few thousandths taken off here or there).
• That's all I can think of off the top of my head.

• Question 1: see photo below. When your gun is in this state (barrel off, Top Lever latched open, Locking Bolt held rearward by the Cocking Lever, is the front of the Locking Bolt basically flush with the breech face as shown?
• Question 2: is that the original Top Lever Dog on your gun, or a replacement?

 

Hi Jeff, are you sure the bolt doesn't move the hammer back slightly at the end of the stroke? I seem to recall that being the case to free the firing pin fron the primer divot so the barrel can pivot but may be thinking of a different gun.

And KS-5, can you measure from the front of a new block to the back of the dog slot to see if Browning perhaps took a little off the original block?

-Scot

 

Thanks for the reply Jeff.

First thing, I finally did get it to do it with the hammer cocked, but I had to work at it. Top lever Dog is original.

Second, the top lever locked over fully, the bolt is actually more retracted than the picture you have (pictured from above, below). I can not see that little rim on top from above. It is all the way back in from the face completely. Like I say though, when it locks up, and I move the barrel slightly up and down, I can feel that movement in the top lever. When I push the top lever over past the locking bar stop basically, it releases and allows the breech to open fully.

Third thing, the hammer is just starting to cock backwards when this stops from opening. (Pictured) I have tried every combination including the original cocking bar, with two new bolts, and a new cocking bar with both new bolts. Same results.

Fourth thing, the new locking bolts (2) face is slightly angled forward toward the bottom. (Pictured from above). Then the angle that contacts the barrel lug goes back from there. Your locking bolt pictured looks pretty square. Which is what I remebered from past BT-99 bolts.

I can see the barrel lug hitting the bottom of the bolt, when I peek down into the opening of the breech, which seems odd. Why is the barrel lug moving back into the locking bolt, with the pivot tight in the hinge. I understand the angles part of the pivot, but usually the lugs are tapered or rounded to account for that. Then also, why is it locking up, instead of pushing the bolt backwards. Only thing I can think of, is it is pinching the bolt upward at such an angle, that it is creating enough friction to stop it from pushing it backwards instead.

This is really irritating. I have had this receiver completely dismantled probably fifteen times, with all of these combinations, and analyzing.

One more thing, I tested this gun (snap cap) before I gave it back to the customer. Absolutely no problems when firing, and opening ten times. Gave it back to the guy and he shot a round of fifty. Came back and told me it was doing this more than half the time. I think the other times it did not do it, he actually held the top lever over past the locking bar stop.

This is frustrating. I gave him back the old locking bolt. Otherwise I could compare, and do some measuring.

I believe the BT-99 Max is the same bolt as a standard BT-99 correct?

 

I'd send it to Phillips Gun Smithing.

 

I am coming up with 1.460". Here is a diagram of what looks to be happening. The barrel lug seems to be squared, compared to other lugs (Red). I know we are only talking .002" or so difference, but why it is so defiant to open is a surprise, if it is such causing the problem. Why won't the bolt just push back out of the way that .002" with some force. I am thinking it is not a bolt problem, even though it is coming in contact with the lug when opening.

This diagram is not to scale, or exact alignments.

 

@KS-5 Spec. For your photos, I'm sure you simply hadn't yet re-tightened the set-screw shown below, but don't forget it when doing final re-assembly. It couldn't really be the cause of your original problem, but I thought I'd mention it.

I am thinking it is not a bolt problem . . .

Yeah, I tried to envision what else it could be . . . some burr on the inside of the receiver? Front fork of the new Locking Bolt interfering with barrel monobloc? Back and forth play in the Cocking Lever? Undersized Cocking Lever cross-pin? Loose barrel hinge pin? Some play in the Top Lever/Top Lever Dog? Something wrong with the Ejector Trip rod? But all these theories sounded either like: a) a remote possibility . . . or b) they would have been occurring all along even before you installed new components.

So, when you figure out the culprit, let us know!

 

OK, I think I know what this is about. When I took this gun in it had been shot out basically. Never had anything replaced since new. This gun is worn good, and had not been maintained real well. The top lever screw was slightly loose, the ejector extension was broke, the ejector retaining pin was broke, the springs were very weak, and the bolt was worn down, along with the lug. This guy said to just get it solid again, so it does not rattle. "Not much time left to shoot, and nobody to leave the gun to". The way it was acting was it would clang closed with a rattle, and the lever was left of center, but surprisingly snug. The top lever spring was so weak, along with the mainspring that it would almost fall open, once the lever was pushed. The forend screws were loose also.

So, I took it all apart and put the new bolt in, which I had. I had to order the springs, and firing pin. I had an old set of springs I took out of a gun, but they were way stronger than what he had in there, so I put them in. They looked like J&P springs in a package from them, but I had written used on them. As you know even used J&P springs are stout.

So being the springs were so much stronger, and the guy saves his shells for reloading, this is what I think is happening. When the lever is pushed all the way over the barrel will start to fall, but if you let it retract before the lug lets the cocking/ top lever locking bar engage the bolt it comes back past the locking bar, and is now under the bottom of the bolt, which will stop the breech from opening, because it can't raise up and cock the hammer back. That is why you could feel the top lever moving, because the bolt was rubbing on the barrel lug, and not locked back by the locking bar. Also why the hammer was just starting to cock and move back.

So basically, when I put the new springs in from J&P that extra tension just caused you to push the lever over far enough to pull the bolt, but the extreme tension would push back the top lever slightly, and the extra mainspring tension made it much harder to open at the same time.

I am thinking every BT-99 would be able to do this. Or maybe, being this gun was so worn something is causing this to happen. I know old age is a factor with those J&P springs. LOL

The odd thing is, out of the ten or so rebuilds I have done on a BT-99, I have never had this happen, or noticed it.

So anyway, thanks for all of the time and help. I am embarrassed that I did not figure this out as the reason, but the guy told me it was a problem. I just never knew it was our weak fingers. Also, I never thought that it was possible for this to happen mechanically.

 

@KS-5 Spec. Good detective work!

I just tried the same thing with a brand new BT-99 and was easily able to duplicate the behavior when the hammer is uncocked. This is what's happening:

1. Close gun. Pull trigger to drop the hammer.
2. Prepare to open the gun by moving the Top Lever to the right in the normal manner.
3. Start pivoting the barrel open as usual. But, if you get the barrel just a little bit open and then "lose control" of the Top Lever, the problem will occur. By "lose control", I mean the Top Lever either a) slips away from your thumb, or b) you slightly relax the pressure applied to the Top Lever. The result is a jammed barrel . . . on my gun, it jammed in exactly the same position you showed in your first photo back in Post #1.
4. What's actually happening is: a) operator didn't get the barrel open very far before losing control of the Top Lever, meaning, b) the rear of the Cocking Lever hasn't yet pivoted upward far enough to catch the Locking Bolt and latch it rearward, which means: c) when the user relaxes pressure on the Top Lever, the Locking Bolt is free to move forward again, but the barrel lug is now directly in its path and in close proximity. Result: the front face of the Locking Bolt moves forward slightly and jams itself against the barrel lug as shown in the illustration below. I verified this under strong light and magnification. This explains the marks you were seeing on your barrel in this area. It obviously jams even harder with a new, stronger Top Lever Spring like the one you installed.
5. Cocked vs Uncocked Hammer: as you know, when the hammer is uncocked, the bottom nib of the hammer is applying strong pressure to the notch in the Cocking Lever . . . the Cocking Lever is practically locked in place until you start pivoting the barrel open and leveraging the hammer back to recock it. When the hammer is cocked, the Cocking Lever is unrestrained and can easily pivot on its cross-pin axle. This explains why it's very difficult to reproduce the problem with the hammer cocked . . . opening the barrel requires much less effort since you're not re-cocking the hammer, plus the rear of the Cocking Lever moves upward much more easily to catch the Locking Bolt and properly hold it rearward.
6. Final conclusion: there's nothing wrong with your BT-99. As you said, the problem is basically 'operator error' (with any BT-99, you must make sure to hold the Top Lever fully to the right until you get the barrel open a sufficient amount to have the Cocking Lever latch the Locking Bolt rearward). Worn/loose parts could make the error more likely to occur as you experienced. And of course, holding the Top Lever fully to the right is a requirement of just about any break-open gun. A shooter with weak hand strength may have more difficulty here.

 

Guys - great thread!!! Very helpful and informative.
Thank you.