Base expansion

[6BRinNZ]

Is this the correct place and tool to measure base expansion?

 

[Minesweeper3433]

Thats how I do it but I never bothered to ask so now I am questioning myself. Can't wait to see the other replies this gets.

 

[BoydAllen]

It depends on what you are using the measurement for. If you are keeping track of how much your FL die sizes the back end of your case, that position and tool would probably be fine, although I see no reason to have the anvil below the top of the extractor groove. On the other hand if you want to check the expansion of the solid head of the case, as a way to estimate the peak pressure of a load, the best tool is a blade mic. that reads to .0001. The measurement is taken a t the bottom of the extractor groove. This is probably best done on the second firing of a case, since case heads harden with repeated firings and sizings. When I have done this kind of measurement, I made a mark, with a Sharpie across the head of the case so that measurements would be taken at the same location.

 

[6BRinNZ]

Thanks Boyd. I want to check on base expansion....hmm so the recommendation is to get a blade mic?
I was hoping to not have to shell out for the blade mic...the only reason being I don't plan on using it much just whenever I work up a new load which isn't frequent in a 6BR. Even then I don't normally bother but with the 105 Hybrids the only node I hit was at the top end of powder load so in this case I just want to double check all is ok.

 

[BoydAllen]

I use more conventional methods to estimate pressure. I usually do a pressure series, starting low, with the bullet near jam (max seating depth without soft seating) one shot per load, FL sized brass, increment suitable to case capacity, looking for ejector marks, at primers, and evaluating bolt lift. For example, for a PPC it is usually bolt lift that stops me. I usually do this loading at the range, often reloading the same case. That way, if the primer pocket starts to get loose, I will pick up on that.

 

[6BRinNZ]

I use more conventional methods to estimate pressure. I usually do a pressure series, starting low, with the bullet near jam (max seating depth without soft seating) one shot per load, FL sized brass, increment suitable to case capacity, looking for ejector marks, at primers, and evaluating bolt lift. For example, for a PPC it is usually bolt lift that stops me. I usually do this loading at the range, often reloading the same case. That way, if the primer pocket starts to get loose, I will pick up on that.

Yep - same - I have found that a 6br in a Barnard P action with lapua brass and CCI 450 primers the usual methods are not good indicators. The same thing in my Howa action I can reliably use primer cratering and sticky bolt lift. with sticky bolt lift being the primary factor.

Comparing primer seating between the loads in the Barnard which is at the top end and loads in the Howa which is mild I can detect a difference but the primer pockets are easily what I would consider tight. If I didn't have a side by side comparison I probably wouldn't notice a difference. The cases have been fired 3 times at the hot load. So all up I don't think I have much expansion but I figure measuring it is the best proof. If it was a varmint load I probably wouldn't worry but as this is for F-class I want to do double checks.

 

[BoydAllen]

I usually don't consider cratering. So many firing pins are too loose in the holes in bolt faces. I look at how sharp the corners of the primers are, and if they are exactly flush with the case head. If you trap a fired case between the jaws of calipers, after cutting any crater flush with a file, you can hold it up to the light to see if there is even a tiny gap that indicates that the primer is sitting above the head. If it is, it may be warm, but it is not hot. This is more common than you would think (a tiny gap). Some actions can be used with hotter loads than others. I run pressures in my Viper with Lapua brass that I wouldn't use with American brass in a factory action. On the other hand if you really want to measure head expansion, start looking for a blade mic. on ebay. Just remember that I hot load worked up in cool weather, will be too hot in warm.

 

[6BRinNZ]

Thanks - I think I will go down the blade mic route. Weather - yes last year I had a .5 moa POI increase due to temps, although typically I would only shoot in 10 deg Celsius increase from when I work loads to the peak of summer, so maybe not the extremes that occur in other parts of the world.

I understand that with a blade mic expansion should not be greater than .001" (as a rule of thumb)?

 

[alf]

My final test on match loads is to repeatably load the same case by neck sizing only and see if/when the primer pocket gets loose and the case starts to chamber hard.

The last two Dasher barrels, I quit at 15 times with the first, with nothing happening, so I quit, the other, the case was starting to get snug after 12 firings, but with no pocket problems. These were both with 105's at 3060.

 

[BoydAllen]

I do not think that you can monitor old cases, using a blade mic, to reliably track pressure as the temperatures change. I think that this method is mostly useful to determine the safety of loads. I think that targets are the only way to really keep track of tune. One thing that I seem to see on the internet is the idea that once one has found a load that works that it is "the load". In short range benchrest, typically shooting a PPC. Shooters have all of their loading equipment with them at a competition, and most of them reload cases between matches, adjusting their loads as they feel it is necessary. I know that for the longer ranges this is not typical, partly because of the pacing of the matches. In any case, I think that for that kind of competition, I would make sure to test loads (loading at the range) in all temperatures, and chart the differences in powder charge that were necessary to stay in peak tune as temperatures changed throughout the year. If a competitor travels to matches in different locations, that are at different altitudes, that will also have a considerable effect on tune as well. In any situation where one can load where one shoots, a lot can be learned, even if it can only be done when practicing.

As to cases getting tight after 15 firings, this makes me wonder if the FL die has been reset to maintain shoulder bump as cases work harden and have more spring back than when new, or if possibly the die is a little too large for the chamber.

 

[6BRinNZ]

I do not think that you can monitor old cases, using a blade mic, to reliably track pressure as the temperatures change. I think that this method is mostly useful to determine the safety of loads.

Sure - just to clarify for what I am after - I will take a new case, measure it and then load and fire it with my predetermined load. In this instance I understand that .001" expansion or less should be acceptable. I am working on - expansion closer to .001" while not conducive to case longevity is still considered safe.

 

[BoydAllen]

Let me tell you a story about one of the only times that I have used a similar method to what you propose to do.

Years back, a friend who is a meticulous reloader asked me to order a reamer for a short throated (so that he could reach the rifling within the confines of the magazine) tight neck (requiring necks to be turned) chamber, in 300 Weatherby. After the rifle was finished, we went to the range, to do some careful testing. Since there was no information available, and I knew that the shorter throat would cause more pressure than the usual Weatherby freebore, we started with a middle of the book load, using the slowest powder that would still give top velocities. I did all the testing with a single case, that I marked with a Sharpie across the diameter of its head, so that all of my measurements on the belt would be in the same place. The first shot on the case, moved the belt, but there were no other signs of pressure. From that point we kicked the load up a half grain at a time, seeing various pressure signs as we went. Since the cases are manufactured by Norma, and known to be a bit soft in in their heads, compared to American brass, and since the diameter of the spring plunger ejector was on the large size, we kept on going beyond the point where we saw an ejector mark (I am just reporting here, definitely not recommending.), which eventually took us to a load that was past the top load that was listed in the manual. About the seventh or eigthth firring of the case, after I had noticed a slight loosening of the primer pocket. I measured a second increase in the diameter of the belt. Before that it had not moved, except for the first time. At that point we stopped, noted the temperature, and made a note that our top load would be a full grain below that. This was done in the heat of summer, in central California, with the bullet seated into the lands, with a rifle that would only be used in much cooler weather, for out of state elk hunting, on a private ranch just below the Colorado, New Mexico border. The belt was measured with a decent 1" mic. that measures to .0001. Another thing worth noting was that even though the neck and throat dimensions were tight, I left the body diameters the same as factory. I had a reference drawing. The whole test was done neck sizing, and at the end, after all that pressure, and repeated firing, I could chamber that case in the rifle without any tightness. Sometimes when shooters want accuracy, they assume that a tight fit everywhere is the way to go. The reason that I did what I did on the body specifications is that I learned a lesson from a friend who has a very accurate 6BR that started with a chamber that was too close to the size of a FL sized case in the body, causing a sticky bolt with loads that were only warm, When he had the rifle chamber reamed with a slightly larger reamer, the problem went away, Neck and throat dimensions were the same on both.

Getting to the important part, when the rifle was taken on its first hunt a shot high on the shoulder of a good sized elk dumped him legs up on the spot, at a fairly long distance. Needless to say, everyone was happy with the result.