My back-up singles gun is an 870 TB that Remington manufactured in 1965. The barrel is 29.5 inches long, but I do not believe that it was ever cut (I inquired at Remington about the unusual barrel length, and based upon my description of the gun, serial number, etc., they confirm that it was manufactured as a trap gun). It is choked full with a vent rib.
I shoot the gun generally well, but recoil is a problem, and it just does not seem to pattern as well as my 870 TC.
I have a question about lengthened forcing cones:
While I would like my local gunsmith to lengthen the cone on the 870, another gunsmith has warned me about a phenomenon called “blow-by.” He maintains that tampering with the forcing cone on a shotgun barrel will mitigate the gas seal between the barrel wall and the wad. Consequently, he condemns the idea of altering the forcing cone in any way.
Are you familiar with this concept, and if so what is your recommendation?
Thank you for your assistance.
Aha! You’ve been reading my old articles. That’s risky business. I don’t dare read them myself. What I have said previously almost invariably conflicts with what I say today. 9/95 was sooo long ago that the laws of physics have surely changed since then. Of course, I put all that in there just to keep people entertained.
Your 29.5″ TB really does sound like an odd barrel length. Not that I don’t trust Remington, but if you aren’t getting the patterns you want out of it, I would certainly get the choke miked (measured by a bore micrometer at your gunsmith’s) to see what you have in the way of constriction. Take your TC with you, get that measured too and compare the constrictions.
You don’t mention what your pattern differences are between the guns. What percentage does the TC throw and what do you get with the TB? What many people describe as a poor pattern is simply the gun shooting to a slightly different point of impact. Your TC may be dead on and smoking them with a 75% pattern, while your TB could be shooting to a slightly different point of impact with an 85% and yet giving you thinner breaks. That’s why you have to put it on paper and test for BOTH pattern and point of impact.
I’m just guessing here, but the fact that you find the TB to be harder shooting than the TC as well as having an inferior pattern, might just indicate that the guns are stocked slightly differently. This would give you a different point of impact with the TB and might also change the way the gun recoils. You don’t mention whether the TB’s abusive recoil is in the form of face slap (which is almost always a gun fit situation) or shoulder/neck/head misery (pure knee-buckling, thumping, headache-pounding recoil).
As to forcing cones and “blow-by”, your gunsmith is absolutely right. It can happen. Under the right conditions of cold weather and inferior plastic wads or old fiber wads, ultra long forcing cones can permit gas to blow around the wad skirt. With modern shells,the warmer the weather and the higher the quality of the plastic used in the wad, this is less likely to happen. As I have mentioned before, manufacturers have very different attitudes on cones. Browning, both Belgian and Japanese, stick with quite short cones (though the Japanese guns are overbore). Beretta standard guns have longer cones than the Brownings, but not really long ones. Krieghoff’s popular target guns have considerably longer cones, but their big dollar pigeon gun, the Ulm, has short cones. Go figure. But none of them have super-long (5″ -6″) cones.
The problems arises with the “if some is good, more is better” syndrome. All shotguns have two forcing cones per barrel. One is the bore taper leading from the bore into the choke at the front. We will skip that for now. The other is the taper in front of the chamber leading into the bore. This latter cone is what most people refer to when they talk of forcing cones. Normal 12 gauge forcing cones are in the 1/2″ to 3/4″ long area. This is usually measured in degrees, but everyone refers to it in inches. Until recently, cones have been kept “short” (i.e. standard length) so that there would be no blow-by or gas leakage with the old fiber wad shells (or the new environmental degradable fiber wads).
Modern plastic wads (made of new, not recycled, material) obturate quite well and have no trouble sealing in most forcing cones as they taper from a typical .800″chamber to a standard .729″ bore over the distance of an inch or so . The whole reason for short cones is that inferior wads don’t seal very well and the designers want to get the transition over with fast to minimize blowby, hence a short cone. A little bit of gas leakage is tolerable, look at cylinder/barrel gaps on any revolver.
Stan Baker, the respected Seattle gunsmith, did a great deal of experimenting with lengthened cones and settled on an optimal length of about 1-1/2″ to 1-3/4″ inches. He felt that cones longer than that contributed nothing. The respected Krieghoff clay target guns have all been made with long cones in the 1-1/2″ to 2″ area and I haven’t heard of any problems associated with those over the years. The problem seems to come when you go further. Many reputable after-market machine shops will run your cones out to 5″ or 6″ because you earn money in business by giving the customer what he wants, whether he knows what is good for him or not.
Since this is turning into a book, I’ll tell you a story. I once owned a 32″ Belgian Browning Pigeon grade trap gun. For reasons which still escape me, I sold it to a friend. He felt it kicked, so he had the forcing cones lengthened to super long 5″ by a reputable shop. The work was first class. Shortly after that I was shooting with him and noticed that his gun was “blooping” occasionally. He was using cheap Remington promo ammo. When he switched to contest quality Remington STS, the blooping stopped. My guess is that the wad in the cheap ammo wasn’t sealing in that long cone. Perhaps it was a bad lot of plastic or wads, perhaps the weather (cool, not cold). I’ve produced the same blooping sounds with an off brand wad in a gun with normal cones, so I’m not 100% certain to blame the cones. Still, my medium length coned Beretta 303 shot those Remington promo loads just fine when he gave me the rest of the ones he had!
Why do people bother with lengthening forcing cones at all? The theory is that the longer the cone, the less constricted the transition is for the shot when it travels from chamber to bore. It’s the abrupt funnel vs the long funnel analogy. The argument is that the short cone jams and distorts more shot than a longer one does. Not only does this jamming damage the shot, but it also adds a bit of extra shock which is translated as recoil.
In the real world, I’ve had cones run out to about 1-1/2″ on a couple of guns. On one of the guns I could detect the very slightest reduction in felt recoil. It was almost too little to measure and maybe I just wanted to feel less after spending that money. On the other gun, I couldn’t tell any difference. I couldn’t detect any change in pattern, but that might be because when I had the cones run out, I also had the guns screw choked.
That’s one of the problems. When people get cones run out, they also often do other barrel work such as porting or backboring. This muddies the water.
One final thought: why don’t you just switch the barrel from the TC to the TB and see if that makes any difference. If it doesn’t then you can be pretty sure that you have a gun fit difference between the guns. See, all that drivel I wrote and I could have just suggested that first. Perhaps it has warmed up enough by now to go shooting.
The Technoid writing for Shotgun Report, LLC