doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Proof failures

miller,

What you mentioned about the choke acting as a sort of "check" seems to the logical mind to be so. However, we have been told that small shot acts like a liquid, and that as it enters the choke it actually speeds up, being subject to the venturi effect. We see other applications of the venturi effect around us, carburetors, cooling towers at nuclear reactors, why airplanes and hydrofoils fly, etc.

I, however, am not convinced that a load of shot acts the same as a moisture laden gas, and will continue to believe that great stresses occur when the shot enters the choke constriction area. Would you not describe the passage of shot through the reloader as flow? Probably, shot doesn't act exactly as a fluid, but it certainly does so in many ways. If the choke is not responsible for a venturi effect reduction of pressure within the shot column, then to what do you ascribe choke effect?

The greater the constriction the greater the stresses, I believe. This line of logic should lead you to believe that the sharper the choke taper the higher the stress. However, choke taper angle is not much of a concern in choke performance. Rather, it is the total constriction. In a very sharply tapered choke, the shot charge could be aware of the total constriction. However, in a shallow taper, the shot charge could all be contained within the choke section and have no idea of the final diameter.

What I would find interesting is how the bulged barrels in question were choked.

Stan

Stan

Don, If there is a reduction in pressure within the shot column it is just a physical side effect of the forced lengthening of the shot column (disagree as the lengthening of the shot column comes with increasing velocity which requires reduceing pressure), emphasis on "forced", because all of it simply cannot get through the constricted area in the same length that it was before entering it (agree, but I believe the shot must obey fluid flow laws and acts mostly as a fluid in a venturi). Lower pressure within the shot column does not assume no stress is transmitted to the barrel at the choke area (Stress on the choke area must be the hoop stress due to pressure within the shot column and the following gas column. I do not believe the shot is transmitting momentum to the choke walls.). If indeed there is a lowering of the pressure within that shot column it comes at the expense of great outward force which is the result of the column being forcibly lengthened (Disagree. The lowering of the pressure within the shot column is in exchange for the shot speeding up to maintain mass flow rate.). What I meant was that I do not believe that shot necessarily SPEEDS UP while passing through the constricted area (choke, venturi) [Yep, this is well established by Remington data from the '30's - one fps per 0.001" constrictionis a fair rule of thumb.]. This is what I was taught in physics was meant by "venturi effect", the acceleration of the gas or liquid [Venturi effect is acceleration of the fluid in exchange for reduced pressure. Venturies have been used on airplanes as a sorce of vacuum power for instruments]. I am not entirely dogmatic on this point, however, as I cannot prove it either way. If it has been proven that shot accelerates in a choke, I stand corrrected on that point (Remington has published proof that choke increases shot speed). Sure, shots flows, but not the same as a gas or liquid, IMO. The shot flowing through my reloader is not under several thousand psi, either (Just gravitational pressure of the column). Of course no outward pressure is being exerted as the shot enters the drop tube on a reloader (Disagree. Not a lot, but there is pressure due to the weight of the column due to gravity) , it is only being acted upon by gravity. It isn't hitting a constriction at 1250 feet per second (if it does flow, it doesn't actually "hit" the constrictiion).

The length of choke taper has been debated and argued for many decades. Many different makers had their beliefs/ideas about this. I have my opinions as well, and opinions are all I claim them to be, not fact. However, if the sharpness of taper has no effect on choke performance, why not just try cutting a .040" X-full choke with a taper length of only .040" (Not exactly no effect, but over usual taper lengths, say 1" to 7", no appreciable effect. Very short or highly constricted tapers would get into the sonic speed of the shot, not the sonic speed of air, but of the shot and I don't know what that would be.)? This is an extreme example, just to make a point, because hopefully you don't believe that the stresses on the barrel steel in the radical taper would not be greater than it would if the taper took place over a distance of 3/4 inch, or more (Would depend on some factors I don't know. Remember Greener's experiment with a super thin choke wall.).

I did not make my point clearly enough in the earlier post, when I said that I would be interested to know what the constrictions were in the bulged barrels, that I was relating it to the earlier thoughts that possibly the proof houses had begun using steel shot proof loads. I certainly don't think that tight choke alone should cause a sound barrel to fail proof with a lead shot proof load. But you don't really believe that all shot, even that of very hard material, flows through chokes without causing stresses to the choke area, do you? Even steel? I think you know how risky it is to shoot steel shot loads through very tight chokes. I have seen any number of ruined chokes and barrels, at a friend's choke tube business, that were ruined by using steel or harder non-toxics through very tight chokes. The venturi effect may have eventually lowered pressures in those shot columns, but it was at the expense of a barrel and a choke tube. I think steel shot is more prone to bridging due to larger shot sizes and higher inter-pellet friction of steel on steel. The bulge comes from a gas hammer and scoring can occur as the end pellets skid against the barrel walls. Stan