More about throat erosion

[John Weigel]

Although I didn't get a lot of interest in my post about the scary-fast throat erosion I've been experiencing since switching from jumping 180 hybrids to VLDs 'jammed' 18 thous (.018"), I did end up talking about it at length with several shooters at recent comps. Here is a rough table showing rate of receding contact with lands I've experienced with one barrel, this one in SAUM, with approx 300 rounds shooting hybrids prior to the 12 Dec starting point below. I'm still trying to give the VLD change a fair shake, though I did like the 180 hybrid methodology with a starting jump of something like 24 thous, whereafter periodic adjustments of tuner could recapture tune with minimal effort - 'chasing the lands' wasn't a thing. My current batch of 2209 allows 55.2 gr for about 2920ish fps with this SAUM barrel.

The following measurements are of course relative numbers, as per my gauge. I use the Speedy Gonzales promoted method https://www.youtube.com/watch?v=AFiIuzvgHYc because a) I haven't as yet ordered Matt P's amazing new lands-assessing gizmo, and b) its too hard for me to remove ejector without losing spring, to do the Erik Cortina promoted method https://www.youtube.com/watch?v=a7vjgEgnhHk .


12 Dec 2.307"

67 shots

11 January 2.304"

56 shots

3 February 2.310"

78 shots

19 February 2.328

Erosion during the period in which 201 shots were fired was .021", or approx 1 thousandth inch erosion per ten shots. If this was the case with jumped hybrids, I wouldn't worry - as occasional tuner readjustment has worked for me in the past. But with the so-called jammed VLDs, I don't like the idea of flirting around the 'just 'touching' and 'almost touching' threshold, as in this admittedly small example, waning accuracy definitely returned each time I've adjusted seating depth to return to 18 thou into lands.

John W

[Rich4]

I have had specified to me many times, Jammed with capitals in bold proper square rifling marks, or jumping, not just touching.
I also start any work up just touching on the assumption it will walk away rapidly and then work back.
I do subscribe to Cortina’s methodology of ignoring the actual jam point for the same reasons, it’s always moving forward and the contact points are changing shape.
Mating tapers are ridiculously difficult to measure distances from without erosion.

[Walt]

Interested to hear the method / technique you both use to measure OAL.

[GuyLuke]

I watched the method when the guy removed the barrel and kept seating deeper until the projectile came out of the bore without 'sticking'. Seems better than Eric's method of seating the bullet using the bolt which could introduce error through projectile deformation due to the force applied. Instead of removing the barrel could one not tap the primer of a case 1/4" UNC and use some threaded rod to do the same 'stick test' without having to remove the barrel?

[Rich4]

I use a Hornady tool, the bent one with a wire extension.
Multiple times averaged, the technique is more important than the tool, like any precise measurement.
I’ve tried Cortina’s method too, it’s logical however only as far as allowing for your particular headspace, which only needs measuring once to allow for the hornady case unless you make your own.

[John Weigel]

Interested to hear the method / technique you both use to measure OAL.

Hi Walt. I don't measure OAL, just case-head to the 'near ogive' of projectile indicated with my particular (SAC) gauge and calliper. The other relative measurement is that forced upon me when seating projectile: the length from case head to 'near tip' of projectile that unfortunately translates bullet shape inconsistency into inconsistency in ogive to lands relationship in practice. I've whinged about this inherent problem in our usual seating practice, and have so far failed to customise seating stems to push bullet from closer to ogive, because I'm a lousy machinist. What I do instead, is load say 50 rounds, separate these into three groups - usually varying 3-4 thous as measured with my gauge, and shorten sizing die incrementally to get all the rounds to within 1 thou of intended measurement. Yeah, I know.

I think the only smart use of OAL is when measuring OAL of projectiles, prior to pointing to ensure similar forces in pointing for consistent as possible bullet deformation. OAL of entire cartridge instead of measurement to (nearly) ogive will unfortunately pass along the variation in projectile shape, meplat deformities included. I think this level of hand-wringing is not so relevant if you are jumping, and the longer the jump, the wider the accuracy node (at least that's what I've found). On the other hand, when I've experimented with 'jamming', there is noticeable in difference in POI and group-size depending on how deep the jam. EG I found that there is a sweet spot between 10 and 20 thousandths beyond contact with lands. Things start going funny after 30 thous, and in the no-man's-land area of +5 to-5 contact area.

John

[MarkS]

Interested to hear the method / technique you both use to measure OAL.

The best method hands down
Mark Gordon's from SAC method as shown here by Cal at PRB
https://www.youtube.com/watch?v=cAVDu-4gLrk

Read more here
https://precisionrifleblog.com/2020/04/ ... ment-tips/

[BATattack]

I commented in the thread regarding powders and throat erosion but didn't get an answer. What brand of barrels are you using?

I remember a brand that had a bad batch of steel and the barrels were toast at 500.

[John Weigel]

I commented in the thread regarding powders and throat erosion but didn't get an answer. What brand of barrels are you using?

I remember a brand that had a bad batch of steel and the barrels were toast at 500.

BATattack,

All three barrels I’m playing with are Bartlein, not purchased in same batch. The high rate of erosion is in common with all three. I suspect other shooters are having similar results but don’t know it because it’s a pain to properly monitor. Was hoping to elicit a few responses from people with same or different results. Can’t believe it’s just me. Only odd variable might be that I switched to White River primers prior to commencing switch to VLDs.

[Matt P]

Hi All
I gave one of my SAUM barrels a hair cut this morning (moved the chamber forward a couple of turns). So while I was there I measured OAL on a new chamber, this thread dropped into my head so I removed the barrel from my other rifle (chambered with same reamer ) which has done minimum of 500 rounds (probably more as I don’t keep accurate records), the results were surprising after reading these post. There was a total of 0.008” movement of the OAL. I have very accurate method for measuring so the numbers are correct. It’s a Bartlien 9 twist 4 groove chamber in 7mmSAUM Imp, using 59 grains of Re23. It’s a sample size of one, so take it for what you paid for.
Matt P

[Frank Green]

Just my opinion and a couple of observations.

When I did shoot Bergers... I like the VLD's way more than the Hybrids.

VLD's don't need to be jammed to get them to shoot. It's like anything else... seating depth will tell you want your set up wants and will vary from gun to gun, barrel, chamber etc...

I've seen bullets including VLD's up to .060" off the lands shoot better than touching or jamming the bullet into the rifling. My now lost buddy John and I got into it about this around 15 years ago or so. He always jammed his Bergers .020" into the rifling on his Palma rifle. So after our polite argument/disagreement he ran a test at a 1k yards and shot the gun from the bench. He shot groups with changing the seating depth .005" at a time in the same range session. 10 shot groups. His .020" jam gave him 3/4moa for groups. At .020" off the lands the gun had dropped to 1/3moa as he got to .040" off the lands his groups had gone back to 3/4moa. So he ran that gun/barrel for the longest time at .020" off the lands.

Regardless of bullet/load etc.... I start a new set up at .010" off the lands and go from there. Rarely do I go any deeper than touching the rifling with the bullet. I don't like jamming the bullet.

Another thought/comment that I have to ask to the OP... are you using the same bullet you started with to check the seating depth from when the barrel was new? If not you could be creating an issue that isn't there. Bullets even from the same box/same lot will vary in OAL, base to ogive measurement and not to mention in the same lot... I've seen match bullets and I won't name the maker vary as much as .001" on diameter. You grab a different bullet expect different measurements on seating depth. Jeff here in the shop on his 284win F Open gun... built a 2nd spare/back up rifle. Same reamer etc... checked seating depth on the new barrel with Berger 180 Hybrids. Came up to me and asked... hey my bullets are going .050" deeper into the case? Can you double check me. So I did it.... and basically got the same thing. I asked the same question.... you using the same bullet as your main gun (which that barrel at the time had a 1k rounds on it) he said no... got a new batch of bullets. I had some of the older/previous lot we were both shooting. I measured the old bullets and his new bullets. Old bullets measured right at .2840". New bullets measured .2844" and not to mention the base to ogive measurement was very different as well. That was the issue making the bullets go deeper into the case.

I think the best way to measure throat erosion if you will... is you need a tool with a round disc shape at a spec'd diameter. When you start shooting the barrel the throat area you will lose that sharp edge but that doesn't necessarily mean the throat is changing.

Another method we have to use to inspect ammunition test barrels and we have to measure the throat diameter to make sure the reamers are cutting properly.... we have to ball gauge the chamber/throat. I feel this has a possible draw back but vs using the bullet method this will give a more consistent data/measurement back.

Later, Frank
Bartlein Barrels