Chamber max CBTO and two different bullets - puzzled

[SGK]

I have for awhile become suspicious of my prior attempts to measure the max CBTO of my Blaser 223. Those familiar with this action will know that you can't use the "Wheeler method". Removing the ejector pin requires one to have a brand new one to hand. Plus the straight pull action style doesn't have the feel of a dropping bolt handle. I'd used a 'loosely seated bullet test' before but had become suspicious that bullets were being jammed and then pulled out again on extraction. Anyway so I dragged out a tool I had purchased and not used - the Hornady Max COAL tool. I'd not used this because it is a pain to measure the result properly. I persevered by first measuring the bullet OAL and then subtracting the BBTO measurement estimated with my Forster Datum Dial Kit. This left me with the length from bullet o-give to tip which I could subtract from a COAL measurement and get to an estimated max CBTO for use with this comparator.

The result using the tool surprised me and was markedly different vs my previous loosely seated bullet attempts. I find it all the more confusing when I compare it with a loaded round with a Sierra Matchking 52 grain bullet. (I do not have some of these bullets, just this one loaded round.) I'm probably having a brain fart but I'm confused by the following.

See attached pic. Sierra Matching round on top. Hornady modified case with 52 gr ELD-M sitting in it at the length to 'touch' in the chamber.

If I chamber the Sierra round with the bullet marked with a Sharpie I don't see any sign that the bullet is jammed in the rifling. So it is either nestled close to touch or has some jump.

You can see where the Forster Datum Dial touches the bullet. Given the ELD-M is more pointed the comparator hits higher (away from the point) than the Matchking. What I don't understand is why the ELD-M doesn't sit at a longer CBTO. If we assume the Sierra round is sitting at touch the rifling would be touching both these rounds at the same point. Given the ELD-M is pointier than the Matchking the rifling contact point must be to the left of the lower comparator point. Yet looking at the shape of the MK I wouldn't expect it to touch so far away from where it hits the comparator. Maybe it's just deceptive.

I've repeated the COAL measurement with the Hornady tool many times with the same result. If it's right my previous estimates of max chamber CBTO were off by about 122 thou!

(I believe the Forster comparator dials are 10 thou less than their nominal calibre i.e. 0.214" for 224 cal.)

 

[Ralph Littlefield]

If I understand your question, you are wondering why the comparator marks are not the same distance from the case head, is that correct?

 

[LVLAaron]

Are you concerned that different tools are giving you a different reading? They should. They're only for relative measurements.

 

[SGK]

No my question/suprise is more about the relative profile of each bullet. Looking at the shape of them, I'm not at all surprised to see the comparator touch closer to the tip of the Sierra. But I'm very surprised that seating at 'touch' would not seat the Hornady longer than the Sierra. The rifling must touch both bullets to the left of the point where the comparator tool touches the Hornady (else the Hornady would indeed seat much longer) - extrapolate that line up through the Sierra. Visually I'd have expected the rifling to touch the Sierra much closer to where the comparator does (and hence the Hornady to seat longer at touch not shorter).

 

[Ned Ludd]

In terms of COAL, whichever bullet is longer from meplat to the diameter of the bullet that first contacts the lands will have a longer measured COAL when the two different bullets are seated to "touching". The CBTOs should be fairly close, if not necessarily spot on. In other words, something like a tangent ogive bullet that has a relatively short nose will typically end up with a shorter COAL, but shouldn't give a radically different CBTO measurement. Sure, there can be bullet-to-bullet CBTO variance. That largely depends on the ogive radius and exactly how flush the bullet ogive fits into the throat/lead angle of the lands.

What is illustrated in the image may be due, at least in part, to a different phenomena. If I understand it correctly, the lines marked on each bullet were not made via contact with the lands, but came from the comparator insert. If so, it is not surprising that the two marks are both A) located at a point not too far above the bullet nose/bearing surface junction where the bullet profile reaches a point that is wider than the comparator insert hole, and B) appear to be (at least by eye) of approximately the same diameter. The phenomena I referred to above is that the holes in some comparator inserts are actually of a noticeably smaller diameter than the point in the throat of the rifle where the bullet first makes contact. Thus, a comparator insert with a slightly smaller hole such as many of Hornady's comparator inserts does not seat at the exact point where the bullet first contacts the lands. In fact, the smaller hole will cause it to seat farther out on the ogive.

In the grand scheme of things, it doesn't really matter because CBTO measurements taken with a comparator tool are often only relative measurements. As long as you're comparing a load with the exact same bullet, the CBTO measurements taken as a part of seating depth testing will be correct relative to one another. Even though the comparator insert may not seat on the bullet ogive at exactly the same spot that first touches the lands, it does seat at a specific distance away from that spot, so changing the comparator CBTO value by a specific increment (i.e. .003") will move the point on the bullet ogive that first contacts the rifling by the same distance. The main caveat in these types of CBTO measurements is bullet-to-bullet dimensional variance in the region between where the comparator insert seats, and where the bullet actually first contacts the lands (see critical distance in the image below). The more bullet-to-bullet length variance you have in that region, the more CBTO variance you'll have between loaded rounds when the seating die micrometer is not adjusted.

The concern I have with your image above is that the two datum points are quite far apart. The size of the comparator insert hole and where it seats on the bullets' ogives probably cannot account for nearly that much difference. I have loaded bullets of dramatically different weights and external dimensions in the same rifle. In general, the CBTO measurements are pretty close, if not identical, whereas the COAL values typically mirror the different bullet OALs. I think this may be the same observation with which you are having an issue, and I can only think of one possibility that might explain it. Is it possible that the red tipped bullet might actually be slightly larger in diameter than the one on top?

If so, I have experienced a similar situation in the past when using the Hornady OAL gauge with a bullet that had an ever so slightly wider bearing surface OR when using it in a rifle with a very tight freebore. In both instances, when I pushed the little plastic stock to move the bullet out to "touching", it would actually hit the beginning of the freebore, which felt like a solid "touch". In other words, the bullet stopped moving at the beginning of the freebore and did not move all the way out to touching the lands. It took me a while, but I finally figured out that my CBTO measurements were short by almost the exact length of the freebore. At that point, I pushed harder on the plastic stick with the Hornady OAL gauge and was able to get the bullet moving again past the start of the freebore, until it reached a true solid touch against the lands. It certainly made using the Hornady gauge a little more difficult with those particular rifles and/or bullets, but with a little practice I was able to get consistent measurements. Is it possible that the tipped bullet at the bottom of your image may have stopped at the beginning of the freebore and not moved all the way out to "touching" the lands? Based on the image, I would not expect the two comparator insert lines to be nearly that far apart. The tight freebore/wide bullet stopping at the beginning of the freebore is the only thing I can come up with that might explain your observations.

 

[Zero333]

2 different bullet ogive profiles mean the comparator will engage each bullet at a different point.

Make sure you're able to push the bullet with out resistance out of the modified case with the tool wire.

 

[SGK]

A couple of opening points: I've used this Forster comparator for a couple of years now, know that it is only useful to compare measurements taken with tis comparator alone and that different bullet shapes will yield different measurements. I'm also not bothered by COAL or more precisely anything forward of the point where the comparator contacts the respective bullet.

Yes I was surprised that there is a big difference in where the comparator hits each bullet but even more surprised at what must be the massive (relative) difference between the bullets in the distance between where the rifling touches and the comparator touches. We know it's to the left of where the comparator hits and that it is (assuming the Sierra bullet is at touch) the same place for both. That means the rifling touches somewhere, for both, in that short distance to the left of the lower comparator marking.

Ned you might be onto something re diameter of each bullet's bearing surface area. I measure 0.2235" for the ELD-M and 0.224 for the Sierra. But if I had a tight freebore this would favour the ELD-M over the Sierra MK...

A tangent o-give bullet will hit the comparator further off the bearing surface than a secant o-give. Such is the nature of the curve - the latter are more 'pointed'. That shallow curve between where the comparator hits and the flat bearing surface must simply allow it to slide further forward before engaging the lands. (By the skin of its chilly chin chin.) While the ELD-M is steep at that point with a rate of change such that the comparator engages quite closely to where the lands do.

Make sure you're able to push the bullet with out resistance out of the modified case with the tool wire.

Yep checked this. No obstruction from the modified case.

Anyway the results surprised me. Particularly that it seems I have been attempting to jam bullets almost an 1/8th of an inch.

 

[Coyotefurharvester]

With your Blaser your best measurements will probably be with the Sinclair bullet seating depth tool. Then you can check each bullet type/brand. Some quick math from an assembled dummy round and your comparator should give you an accurate starting point for each.

 

[Ned Ludd]

A couple of opening points: I've used this Forster comparator for a couple of years now, know that it is only useful to compare measurements taken with tis comparator alone and that different bullet shapes will yield different measurements. I'm also not bothered by COAL or more precisely anything forward of the point where the comparator contacts the respective bullet.

Yes I was surprised that there is a big difference in where the comparator hits each bullet but even more surprised at what must be the massive (relative) difference between the bullets in the distance between where the rifling touches and the comparator touches. We know it's to the left of where the comparator hits and that it is (assuming the Sierra bullet is at touch) the same place for both. That means the rifling touches somewhere, for both, in that short distance to the left of the lower comparator marking.

Ned you might be onto something re diameter of each bullet's bearing surface area. I measure 0.2235" for the ELD-M and 0.224 for the Sierra. But if I had a tight freebore this would favour the ELD-M over the Sierra MK...

A tangent o-give bullet will hit the comparator further off the bearing surface than a secant o-give. Such is the nature of the curve - the latter are more 'pointed'. That shallow curve between where the comparator hits and the flat bearing surface must simply allow it to slide further forward before engaging the lands. (By the skin of its chilly chin chin.) While the ELD-M is steep at that point with a rate of change such that the comparator engages quite closely to where the lands do.



Yep checked this. No obstruction from the modified case.

Anyway the results surprised me. Particularly that it seems I have been attempting to jam bullets almost an 1/8th of an inch.

Certainly the comparator will seat at a different point on the bullet ogive for tangent versus secant ogive bullets. But you'd think that the comparator contact point wouldn't be quite so different between the two different types of bullets in terms of CBTO once they were both seated at approximately "touching" in the case neck. I'm also surprised how far apart the two seemed to be in your picture. A possible effect of a slightly larger bullet diameter and/or a very tight freebore was the only thing I could think of that might explain it. Let us know if you figure it out.

 

[Texas10]

Small caliber barrels seem to be particularly sensitive to carbon or fouling in the throat area when checking CBTO touch measurements. At least that's been my experience. Are you able to verify that the barrel is clean via an endoscope or borescope before carrying out this measurement?

I gave up on using the Hornaday headspace tool as designed, threw away the plastic push rod, neck size the LNL modified case, seat the bullet long and screw it onto the tool, then push it firmly into the chamber and retract. Any tendency to stick indicates the bullet is in the lands. Repeat while gradually seating deeper until no tendency to stick to find the touch point.

Try that and see how it compares to your previous measurements.

As for the Hornaday OAL tool, where the tool touches the bullet has no direct correlation to where the bullet touches the lands. It's just a convenient way to consistently measure the bullet location in the cartridge case. The more telling information for me is to seat the bullet to touch, then hold an second bullet up to seated one and see how far the bullet is intruding into the case.

In your situation, the ELDM is taking up alot more case room and is probably not the optimum bullet to use in YOUR chamber. Your other bullet seats much further out leaving more room for charge and greater velocity.

 

[SGK]

I will take a look tonight with my borescope but I'm pretty sure it's as clean as a whistle.

To your last point, yeah I'm wondering if I shouldn't take this opportunity to switch (back) to the Sierra MK, and move up to 69gr, perhaps with a faster powder such as Viht N530 or N540.