BC And POI Explained, Sort Of


Dear Bruce,

If it was as easy as making up the answers, I would also be a Technoid. The problem is with the Aussie culture. Grab a gun, some No. 7 shells, pay for your nominations, and bob’s your uncle (Aussie lingo). You would be surprised how well this works. Though, this does not apply to the top shooters. These guys have travelled into Europe and the USA and have gathered information from the worlds best. The biggest problem is that these guys, don’t divulge information, unless you hold then down and give then a wedgy. (That normally does not go down well on the sporting clays coarse).

So what does the average fellow do, well that is easy……..get on the net and check out what the Technoid has to say!!!!!!!!

So lets begin. Point Of Impact. What is it? Who Cares? Why is it important? What does it mean in the real world?

As you guessed I am a mechanical engineer, who sometimes finds it hard to come to terms with some of the rubbish that our so called gun distributors feed us. They seem to apply a “smoke and mirrors” approach to the entire Point of Impact question.

History,

A friend of mine recently bought himself a brand spanking new Browning UXS. His first attempts at putting this gun to the test was on the skeet range. Needless to say he could not hit a target. It appeared to me that he was shooting over the top of everything. He did all the obvious things like lowered the stock, stuffed around with this and that, but still had the same problem. He eventually realised that this was not a problem, as long as he ran about a foot under every target. At this point I borrowed this gun for some pattern testing. The gun confirmed my suspicions. It was shooting high. Both barrels 75/25% over rib. I told him the results and he was surprised that shotguns were this scientific. He then went back to his dealer, and the dealer told him that it couldn’t possibly be the gun. He went further to say that the person performing the pattern testing was not holding the gun right and that neither of us were qualified to perform such tests, let alone derive to any conclusions. As for me, I have been called an idiot many times before. Not the first and not the last time, I suspect. As for my friend, being the nice fellow he is, did not argue the point, traded in the gun, wore the $1500 loss (in six weeks) and wrote it off to the fact that Browningâs were crap. (Not necessarily so)

This is where the plot thickens. Another fellow I know is currently having an argument with Beretta, as his sporter is shooting 60% below rib. Though this guy (another under paid engineer) is not your average shot gunner. He is well informed on shotgun performance and often publishes articles on the subject for our local rag. (“Feathers and Fur”) Initially Beretta told him to go and jump, but after a little complaining they agreed to change his barrels. I guess they figured that the bad press was not worth it. Can you image what would happen if only 10% of Australian shooters went out and patterned their shotguns. I make this statement, as I personally only know of one fellow you has patterned his shotguns with any diligence, as for the others “ignorance is bliss”.

This is not just confined to the sporting guns. I was talking to a competitive trap shooter who had to have Briley machine an eccentric choke, to correct his first barrel from shooting low left. Aussie’s are to relaxed, if that was me, I would have screamed blue murder.

Correct me if I am wrong, but in sporting, as we see a good percentage of dropping targets, a 50% P.O.I. would suit 95% of the market. So why do the major manufactures insist on producing sporters that shoot otherwise. For example the Beretta agent recently told me that the DT10 sporter is 60/40%. By the way, I nearly had to beat that out of him.

I am very interested to know more about how manufactures determine & achieve POI and what tolerance they consider to be acceptable? Also, what are the current trends on the international scene? New shotguns are not like cars. You cannot drive before you buy. You can shoot a test gun (if one is available) but once you have paid your dollars, you are well and truly committed.

Regards,

Panos

Dear Panos,

I’m always amazed how much more expert I become the further away the supplicant is! Since you are about as far away from me as can be, I can wax very authoritative indeed (without the danger of you showing up on my doorstep to shake a fist at me).

First of all a definition of terms. “Barrel Convergence” (BC) must be discussed first. BC is the difference in impact points between the two barrels of a double barrel gun. Ideally, BC = 0″. In the real world, BC can be a factor, sometimes a considerable one. In any two barrel shotgun, the barrels must be angled towards each other to get proper BC, much like the machine guns on the wings of the WWII fighters were set to converge at a certain distance. You can’t just set both barrels up parallel as the geometry of an O/Us recoil “wants” to shoot the top barrel high.

The problem with BC is that it is somewhat recoil, shell and shooting technique dependent. It’s also VERY hard to regulate perfectly without testing each and every gun. You’d think that with CNC and EDM machining perfect BC would be a given, but it’s not. Many manufacturers consider up to 8″ mis-convergence at 40 yards to be acceptable and much less than that to be simply good fortune. If at 40 yards your killing pattern is only 24″, you don’t want to give up 1/3 of that to barrels that shoot to different places. Welcome to the world of shotgunning! It’s not precision riflery.

The reason I mention BC before Point of Impact (POI), is that it’s quite possible for a shooter to test his POI with one barrel only to find a completely different POI with the other. It’s quite common for O/Us to shoot their top barrel higher than their bottom barrel. So which one do you test for POI? Aarrgghh! And don’t think BC is just a problem with cheap guns. I’ve seen whopping BC errors (“whopping” like meaning a 24″ mis-convergence at 40 yards) in some very, very expensive guns.

So, let’s assume that your BC is acceptable, or even, miracle of miracles, perfect. To measure POI, I rifle sight flat down the rib. This superimposes the mid and front bead. This may or may not be the way you actually shoot the gun, but we are testing the gun here, not your shooting style. If when you sight flat down the rib, your pattern is 50/50, then you have a 50/50 POI to my way of thinking. That’s a “built-in” 50/50 POI and it’s not shooter dependent.

Of course, when you actually shoot the gun, you may have raised your stock so that you see a little bit of rib when it is cheeked the way you like it. This would make a gun that shoots 50/50 shoot 60/40 or somesuch. Technically, you now have a 60/40 POI, but I think that it’s fairer to still say that the gun is a 50/50 POI and that you, the shooter, have caused the gun to shoot higher by stock adjustment or shooting technique. Same result, but very different cause.

There are all sorts of reasons why people like guns that have different POIs. In trap shooting, some people like a gun with a high POI because they are used to looking flat down the rib and they want the gun to shoot high. Others, like myself, want a gun that shoots 50/50 and if I want to shoot higher, I simply use a higher stock and see more rib. Same result, but a very different sight picture. It just depends on what you want.

One of the reasons that some guns come with stepped ribs that are higher at the rear than at the muzzle is because sighting flat down one of those ribs gives you a raised POI without the manufacturer going to the trouble of building the POI into the barrels. He can take the same gun with a flat rib and make its POI flat. Saves money on the production line.

I’ve always hated sighting flat down a rib, especially at sporting, because the slightest extra cheek pressure such as you use on a dropper puts my eye below the rib and I go “blind”. That’s why I like to see a bit of rib when I use standard cheek pressure. If I get a low bird and tuck in just a bit extra, my eye goes flat down the rib, but never below. Yes, I know you should always use the same cheek pressure for each and every shot, but in sporting if the angles are very low or high you don’t. Skeet and trap are very much easier to control in this respect. I like my guns to be about 50/50 POI when looking dead down the rib so that when I cheek normally and see just a touch of rib they shoot a little bit high, but not too much. I’m not saying that this setup is right for everyone, but it’s what I’ve gotten used to over the years and it’s right for me.

Why do the major manufacturers “insist” on producing sporters that are other than 50/50 when they know that’s what most people want? Three reasons: 1) they are unable to control POI as much as you think, 2) some people like guns that shoot high because they prefer to float their birds, and 3) some manufacturers don’t have a clue. Quick, when was the last time you met a gunsmith who was a really good competitive shooter? Well, in some cases it’s the same with gun makers. Sometimes they just make the guns, they don’t shoot them competitively.

You sell what the customer buys. Note that all the Krieghoff K-80s I’ve seen in the USA have had stepped ribs which are noticeably higher in the rear than at the front. This includes their sporter line. Sure it saves them the expense of fitting different ribs to their guns and the customers don’t seem to mind or notice or care. In theory, that K-80 stepped rib should make the gun shoot high and force the shooter to look flat down the rib rather than see a bit of rib. Perazzi goes to great lengths to sell you any style rib you want. It’s interesting to note that the early Beretta 390 semi-auto sporters had stepped ribs. The shooting public in the US hated them and an entire cottage industry arose to fit the flat ribbed 303 barrel to the 390 before Beretta relented and started to produce the 390 sporter with a flat rib. When the 391 sporter came out, Beretta was smart enough to make it available in a flat rib gun.

I’ve always felt that one shouldn’t attribute to malice that which and be adequately explained by sheer stupidity. That certainly applies to some peoples’ driving habits, but it also may apply to shotgun barrel convergence and point of impact. Sometimes the makers do what they do because they simply can’t control the process any better at the price at which they are selling the gun. Where it becomes inexcusable is when there’s a problem with an expensive gun. I can understand the manufacturers stonewalling when a POI or BC problem arises because there really isn’t much they can do about it without drastically altering the way they make guns. About half the time, they are right to stonewall because the owner is improperly testing the gun. The other half of the time, there is really something wrong with the gun.

One of the main reasons I always prefer to buy a used gun, rather than a new one, is that I can test the used gun for BC and POI and return it if it doesn’t suit me. Those are the first tests I make when vetting a gun.

Bottom line: There is always a potential problem with BC and POI, but fortunately most people don’t bother to test their guns and never notice it. Ignorance is bliss or, more accurately, bliss is ignorance. One thing’s for sure- If you have a gun that you shoot really well don’t, don’t, don’t test if for BC or POI. It’s like going to the doctor’s office when you are feeling fine. All you can do is break even or lose. You can never win.

Best regards,

Bruce Buck
The Technoid writing for Shotgun Report, LLC
(Often in error. Never in doubt.)
<http://www.ShotgunReport.com&gt;

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