First Reload Problems
- Thread starter blueeyes
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blueeyes
Well-Known Member
Cheers
In a word yes, putting more bullet inside the case reduces the capacity available internally and ups pressure, work up from minimum again to be safe.
Well, yes and no depending on the chamber throat ('freebore') length. If a longer bullet is used and it has to be seated deeper because it would otherwise be jammed into the rifling, yes that increases pressures, so a lower charge must be used. If the longer model could be seated to a longer COAL without hitting the rifling, but has to be seated deeper in the case solely to reduce the COAL so that the cartridge fits the magazine, it will not generate higher pressures if all other factors are the same (bearing surface length, diameter, weight, jacket thickness / hardness).
The reason behind the two cases / results is that one of the key factors that determine pressure is available combustion chamber volume, available as in the sense of the remaining space inside the case below the bullet base - but not on loading rather shortly after ignition when the bullet has moved as far as the rifling lands. After ignition, pressure rises very rapidly indeed, but not near instantaneously - that's detonation not combustion and would wreck any action made today. The bullet moves out of the case at relatively low pressures into the throat and is then checked very briefly by the rifling. It is at this point where the bullet stops or slows to an near stop that pressure ramps up really fast and as smokeless powders 'feed on pressure' the charge burn accelerates generating yet higher pressures which accelerates .... Peak pressure depends on the combined volume of the obturated case and the throat below the below the bullet base. If the two bullet pause with an equal amount of bullet shank and tail sitting out of the lands in the throat, they will generate equal pressures (other dimensions and bullet hardness etc being equal). This could apply where the only change between two models from a manufacturer is a hollow point meplat v a slightly shorter nose with a plastic 'ballistic' tip inserted that increases bullet COAL.
(You can tell how low a pressure is needed to move the bullet to that position by simply forgetting to charge the case, Unless very high neck tension and/or crimp is used, the primer alone will move the bullet into the rifling lands, moreover engrave the bullet into them hard enough to need a drop weight or cleaning rod to knock the bullet back out and clear the rifle.)
It is the combustion chamber volume that applies at that point (bullet reaching the rifling) that is the value that determines pressure all other things being equal. That is why a rifle with long freebore or an eroded throat (providing it has been eroded by large numbers of relatively low pressure rounds and has retained smooth surfaces) will produce lower pressures and MVs than loading manuals state. However, rapid erosion producing a very rough surface can have the opposite effect increasing pressures dramatically, but that is a separate issue.
I'll give two examples of how this can work out in practice:
1) comes from the Vihtavuori Reloading Manual No.1 in which a 7.62 cartridge loaded to NATO pressure specs with a ~145gn FMJBT bullet is fired with the bullet seated progressively deeper in small increments with all other components and charge weight kept constant. For the first three or four COAL reductions MV and peak pressure remain constant despite the deeper seating depth, then when it has become a significant drop, both pressure and MV see a reduction.
2) I replicated this myself last week doing seating depth / jump tests with the very long and ultra-VLD form 183gn 7mm Sierra MK in a 284 Win F-Class rifle. Being a very aggressive VLD form I had initially loaded the bullets to be gently into the lands on chambering a round - not 'jam seated' which really raises pressures but being supported enough in the lands to feel some extra resistance on final bolt closure - some 10 to 15 thou' longer than the COAL that has the bullet ogive just kissing the angled start to the lands. This had been tried, pressures were fine; MVs OK if modest, but every group had a degree of vertical stringing, some really badly. American users of the bullet say that jump is needed with it, in some barrels / chambers a lot of jump, despite this being a VLD. So 23 rounds were loaded as follows:
The powder charge weight that had given the smallest group was chosen and COALs were varied as:
3 into the lands as in the first load tests
then four batches of 5 rounds each batch with a reduced COAL by 20 thou' over the initial setting (likely kissing or marginally out of the lands); 10 thou' deeper still; another 10 thou' deeper; finally a 20 thou' deeper increment over the previous 5. So the COALs as loaded saw a 60 thou' variation over the five lots.
Here's what the target (100 yards) and Labradar chronograph said:
Batch 1 (original COAL seated into the lands, 3 rounds) .......... 0.45", 2738 fps, ES 13
Batch 2 (20 thou' deeper than 1, 5 rounds) ............................ 0.9" 2,741 fps ES 17
Batch 3 (10 thou' deeper than 2, 30 thou than 1, 5 rounds ....... 0.37" 2,740 fps ES 9
Batch 4 (10 thou' deeper than 3, 40 thou than 1, 5 rounds ....... 0.65" 2,738 fps ES 11
Batch 5 (20 thou' deeper than 4, 60 thou than 1, 5 rounds ....... 0.27" 2,728 fps ES 12
As with the Viht 7.62 COAL test, reduction in COAL had no effect on MV and hence peak pressures until a certain reduction was achieved that produced a lower MV and presumably pressure, albeit only 10 fps in this case. Why? It is because the increasing amount of jump gives the bullet 'a run at the lands' and when that run becomes large / fast enough the bullet is engraved and passes into the rifling quicker so that peak pressure cannot build as fast or as much before it is accelerating out of the start of the lands and further down the barrel.
I was very surprised though with my own results as I had expected an MV drop between batches 1 and 2 in that being in the lands albeit 'gently' is always expected to increase pressure and hence MVs.
NEVERTHELESS .......... ! This has a lot of theory and maybes in it. Unless you know what you're doing and especially for a tyro handloader, a change of any component and / or setting should be treated with caution, the charge dropped and them worked up again as usual checking for pressures.
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So minor changes provided there's plenty of freebore don't cause a problem but say .15" or 2.29 to 2.14 could / would bump it up? I'm only going on QL showing more pressure for greater seating depth but that won't take into account freebore will bow to your vastly superior knowledge! Thanks for the very in depth perspective.
Is velocity an accurate indicator of peak pressure in the chamber? (genuine question)
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blueeyes
Well-Known Member
well that was a bit over my head but picked up on a few pointers also stripped the bolt and turns out that I have a snake spring in my J lock bolt which is tight in the bolt housing would this be a contributing factor to both hard primer hit and if spring in wrong position bolt lift thanks 
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