Concentricity - setting up dies - runout

[wichita45]

Speaking of smoke and mirrors...The Sinclair gauge depends on the body of the case being round. Many times they are not, and if you set the indicator opposite where the balls support the shoulder, you may get an indication of what I am pointing out. (With my old unit you can do this, at an angle.) OK you say, the H&H also relies on the roundness of the case, but it only "sees" the case body at the back. Of course this depends on a slightly non standard setup, tilting the plastic V block so that it only touches the case at its back edge. Big surprise, if you support a case two different ways, you get different numbers, the implication is that the method that produces the smaller numbers is inferior...says who? Just for fun I got out a loaded, neck turned 6PPC round and measured it with my old style Sinclair tool (three square steel bars on a common central bar, with pairs of steel balls on two). The total range of indicator movement was .002. I then measured the same round on my H&H and i
being round. Many times they are not, and if you set the indicator opposite where the balls support the shoulder, you may get an indication of what I am pointing out. (With my old unit you can do this, at an angle.) OK you say, the H&H also relies on the roundness of the case, but it only "sees" the case body at the back. Of course this depends on a slightly non standard setup, tilting the plastic V block so that it only touches the case at its back edge. Big surprise, if you support a case two different ways, you get different numbers, the implication is that the method that produces the smaller numbers is inferior...says who? Just for fun I got out a loaded, neck turned 6PPC round and measured it with my old style Sinclair tool (three square steel bars on a common central bar, with pairs of steel balls on two). The total range of indicator movement was .002. I then measured the same round on my H&H and it measured .0015, and then I used the H&H to straighten the round to a total runout of* a quarter of* a thousandth. At this point I put it back on the Sinclair unit, and was surprised that it read a TIR or .0015. Scratching my head, I remembered that the validity of the reading was totally dependent on the roundness of the case, positioned the indicator on the case shoulder, (albeit at an angle so the correct measurement would be larger) The shoulder had a TIR of around three quarters of a thousandth. Now it makes sense. Because a standard reading is on the bullet, well beyond where the case is supported at its shoulder, the shoulder eccentricity is magnified to what is arguably a distorted picture of* how well the bullet will be aligned with the center line of the bore. On the other hand, because the point of measurement of the H&H lies between the supports, and relatively distant from any potential out of roundness at the back of the case, and the bullet is for all intents and purposes perfectly round, the more modest needle movement of the H&H turns out to be more representative of what I wanted to measure. In short, bigger is not only not better, it may in fact be worse.

Oh, and one more thing, have you ever tried to straighten a loaded round on a Sinclair Concentricity Gauge? Good luck with that.

 

[Bart B]

Speaking of smoke and mirrors...The Sinclair gauge depends on the body of the case being round. Many times they are not, and if you set the indicator opposite where the balls support the shoulder, you may get an indication of what I am pointing out. (With my old unit you can do this, at an angle.) OK you say, the H&H also relies on the roundness of the case, but it only "sees" the case body at the back. Of course this depends on a slightly non standard setup, tilting the plastic V block so that it only touches the case at its back edge. Big surprise, if you support a case two different ways, you get different numbers, the implication is that the method that produces the smaller numbers is inferior...says who?

Well said; reality's often what we don't want to believe, but it's always best to believe reality. Best accuracy happens when all the real contributors are the same from shot to shot. Which is why I prefer to measure bullet runout by mounting the cartridge in the measuring tool the same way as it fits the chamber.

Bottleneck cases that headspace on their shoulder fit the chamber as follows:

First, they're pushed as far forward as their headspace limit allows by the ejector in the bolt pushing them there. Extractors have enough clearance between bolt face and case rim front edge to allow this. The case shoulder stops and perfectly centers in the chamber shoulder. It doesn't matter how much smaller the case diameter at the shoulder is from the chamber shoulder nor how much out of round both are; the front end of the round centers at that point. With ejectors external to the bolt (Mauser and pre-'64 classic Model 70) and in-line ejectors in the bolt, the case is driven forward hard into the chamber shoulder when the firing pin puts it there usually setting the case shoulder back a thousandths or two. In either situation, the front of the round's well centered up front.

Second, the back end of the case is pushed sideways by the extractor. The pressure ring on the case bears against the chamber wall from the extractor's force. So the back end of the case is a thousandth or two off center relative to the chamber. As both chambers and cases are not perfectly round at this point, the same case will be a different amount off center depending on how it's indexed in the chamber.

Third, because of the first and second conditions, how crooked the bullet is relative to the chamber axis depends on how well centered the case neck is on the case shoulder and how far off center the back end of the case is relative to the chamber. So any round whose diameter at the pressure ring's smaller than the chamber at that point's going to be off center at the back end of the chamber by half the difference in diameters.

So I made a cartridge spinner that has a nylon washer at the front end with a .400" diameter hole in it and a V block at the back end where the case pressure ring touches it. The dial indicator's put a tenth of an inch back from the bullet tip. Rounds are put in the tool pushing the case forward against the nylon washer and holding it down in the V block. I'm measuring bullet runout relative to how the round fits the chamber.

Note that if the distance between the shoulder datum point and bullet tip minus 1/10th inch is twice that of shoulder datum to pressure ring. when the round's chambered there'll be half as much bullet tip offset from chamber/bore center as the pressure ring's off center in the chamber.

I always full length size my belted cases such that their shoulder's set back no more than two thousandths. I don't want those cases headspacing on a belt after they've been fired. After using a standard full length sizing die with its neck lapped out to a couple thousandths smaller than a loaded round's neck diameter, I use a body die (made from the middle part of a full length die) to resize them all the way to the belt which traditional full length sizing dies don't do. That step right in front of the belt needs to be eliminated else the case won't chamber the same each time it's loaded.

 

[Reloader222]

Sorry if I have missed it - how do you use concentricity to your advantage?:

Put cartridges in groups of 3:
Those with 0.001" non concentricity
Those with 0.002" non concentricity
Those with 0.003" non concentricity.

When you rotate the cartridge in the v-block and the dail indicator would show your where the runout is. Then turn it to the same spot and just there when the dail starts returning, you mark the case with a black marker on top of the cartridge. This mean that you have marked the cartridge where the runout is. You do this with all cartridges. Then put them in the 3 groups as indicated above and do'nt mix them.

Then when going to the shooting ranch clamp the gun into the rifle bench clamp. Put up any big target. Now insert the first batch of all 3 cartridges in the magazine with the black mark (runout) of the cartridge showing at 12:00 o'clock. Now shoot the three shots. You would find that the grouping would be very tight and even that all 3 shots could be through the same hole. Now take the second batch and repeat the steps above.

THEN:
Take the last batch with a runout of 0.003" BUT insert one cartridge with the black mark (runout) at 12:00 o'clock, the second one with the black mark at 04:00 o'clock and the last one with the mark at 08:00 o'clock. Now shoot the three cartridges. You would find that the grouping has gone up to 1" or more.

CONCLUSION:
If you load cartridges with the runout in the same direction you could further improve you accuracy. The reason is that you have iliminated another veriable.

 

[Bart B]

If you load cartridges with the runout in the same direction you could further improve you accuracy. The reason is that you have iliminated another veriable.

Some of us folks shooting arsenal match 7.62 NATO ammo in Garands and M14's have done that. We'd shoot ammo with 3 thousandths or less runout in the rapid fire matches. Ammo with greater runout would be indexed with high point up in the ammo box then chambered that way for slow fire events. We weren't allowed to "modify" or "alter" the ammo which marking it would certainly do according to the officials.

Ammo with up to 3 thousandths runout didn't cause any accuracy problems. Over that, there was about 1/4th MOA increase in accuracy degradation for each thousandths runout over 3. Some of that ammo had runout up to 7 or 8 thousandths, but it shot very well when so indexed in the chamber. We put 'em in the ammo box with lowest runout to be shot first then working up to those with the greatest runout then adjust the sights when shots started missing call by too much.

 

[dig]

All:

Thanks again for the continued support on this thread. I am borrowing NECO Gage to tear and compare! Bottom line I think I was doing things pretty right to begin with. I will try the indexing, seems a little tougher in a true Hunting Rifle that gets banged around, loaded, unloaded, horseback etc. . . but will give it a try at the range.

 

[dig]

OK all, been using the H&H Gage for a month plus, compared it to both the Hornady and the NECO Gage. The Hornady is junk the NECO is good and offers options but the H&H certainly does the best job of measuring non-concentricity. Once you get used to playing with the "V" block and index closer to the neck you eliminate most/more of that bottleneck affect some experts commented on. The NECO does not allow this and the Hornady, forget about it. I have now used this on everything from .243 WSSM to .375 H&H. Once you get the set up down its a piece of cake. Great tool.

On another note, I have things down to approx .0025 or less run out in the .300 Weatherby. I tried the marker trick to chamber the same way each time and noticed no difference in group size (three 5 round groups, same lot loading each way). Is that because the run out is so little it doesn't matter? or possibly the free bore in the Weatherby? Maybe a combo of both?

At any rate thanks again all for the help, its surely helped me improve my loads if even incrementally and given me something to play with!

 

[Bart B]

On another note, I have things down to approx .0025 or less run out in the .300 Weatherby. I tried the marker trick to chamber the same way each time and noticed no difference in group size (three 5 round groups, same lot loading each way). Is that because the run out is so little it doesn't matter? or possibly the free bore in the Weatherby? Maybe a combo of both?

It's both. Anything under 2.5 thousandths is fine for 30 calibers. And a lot of bullet jump doesn't help much.