well, if you cannot be bright it pays to be a little lucky.
i was out at the range the other day, and chanced upon a friend who knows his reloading. i mentioned my efforts to get a "whisper" out of the dead stock 6.8mm spc case, given my assumption that since the case is so small is should be capable of doing. before i could even get a hint out that i was having trouble getting the 5744 and 4759 throttled down low enough to go subsonic, because i was having incomplete ignition problems with the small powder charges involved, ... , my friend said, "trail boss."
"but, that is a cowboy action pistol powder," i said.
"used to be," he said, "but, not just for that, anymore, strictly speaking."
here's the deal. old pistol cartridges like the .45 colt were initially fueled by black powder and worked at about 14,000 psi pressure. the old cases required lots of room for the black powder in order to get the performance levels desired, around 900 fps or so w/ 250 grain lead bullets.
today, they are loaded to the old performance levels for several reasons. one, if deference to the old guns that still operate, two is because guns of new manufacture are designed to work at that level of pressure from the cartridge, and three is because those levels of performance are still lethal and provide all the accuracy and performance needed. why break it, if it still works?
a problem developed with cowboy action shooting, a game in which people dress up like cowboys and shoot a lot of targets over a day. people started loading the guns "down," and decreasing powder charges, so they could shoot more rounds comfortably, and more quickly because of the needs of the competition, ... , "more better," as they say. well, it was pretty easy to "double charge" a .45 colt case to begin with, and it got ridiculously easy to "double charge" them with the lighter loads, hence, occupying less volume in an already large case, and as a result barrels got bulged when a weak charge would not clear the bullet down the barrel and another came along behind it, and guns got blown up when inadvertently overcharged.
so, imr powder company (used to be du pont, and is now owned by hodgdon) developed "trail boss."
it is a light for volume propellant, some say fluffy powder, which is designed to completely fill a large pistol case while generating relatively low pressures, to attain relatively low velocities. in other words, the powder companies attempted to make it impossible, or "idiot proof" to load a round to too high a pressure, on the theory that if reloaders cannot get too much powder into a case, it cannot hurt them.
well, idiots are resourceful. it is almost like they go out of their way to blow off appendages by destroying guns with pressure. but, so far, the powder company is winning the contest betwee "fool proof" and "more advanced fool."--
people began putting trail boss into large volume rifle cases which were designed to shoot large caliber bullets made of lead at low velocity. they survived it.
and, somebody discovered that if you put it in modern small caliber bottle necked rifle cases shooting gilding metal clad bullets, you could survive that, too, although you only got low velocity for all your trouble.
fill the case. low velocity. hmmm!!
my friend related the following, from articles that he has read, and as he related were published by hodgdon powder at its web-site giving reloading data. i could not read the hodgdon web-site article because my pdf file reader has taken a mysterious hike, but i found chat room sites discussing it and confirming what my friend related.
reader, beware!!! remember, i am not a ballistician, i am not a reloading manual, and i have no money, wealth, land or anything else of value, nor am i insured up the yazoo, so i am worthless to sue if you blow your fool fucking head into smithereens trying to replicate what i am going to relate and describe. proceed absolutely at your own risk!!!
you're on your own, and good luck, ... . laughing.
but, my friend related, as follows.--
"you cannot stuff enough trail boss into a rifle case to generate any pressure.
"here's what you do. you take your bullet, and you determine how far into the case the base of the bullet projects when it is seated to the proper depth. you mark this on the case. to this point, you fill the case with trail boss, to that mark, where the base of the bullet would be, and that is considered a 100% density loading. then, you fill a trial case to that point, and then pour that amount of powder back out & into your powder measure, and you weigh it to determine your charge weight of imr trail boss. that is maximum, and you will get a very slightly compacted powder charge from that. then, you multiply that by 70%, and that is considered a minimum load, and anything much below that might not get a bullet down the bore and out the muzzle, causing a dangerous obstruction.
"if you want to go subsonic, you have to adjust the load down from there, if the initial load doesn't make you subsonic.
so, i looked at data on the net, and sure enough, the minimum charges shown for imr trail boss were about 70% of the maximum loads, and the velocities of the maximum charges were only showing slow velocities, and low pressures.
so, i determined by the above methodology that using a 150 grain sierra matchking bullet of .277 caliber, i could load about 7 grains of imr trail boss to where the base of the 150 grain sierra matchking bullet would project into the case. i did that by loading a 150 grain bullet into a dummy round w/ no powder or primer, and then measuring on the case to where the base of the bullet projected into the case. i put some powder in another case to that mark, took it from the case, and weighed it on the powder measure, and the powder weighed exactly 7 grains.
friends, this takes some care. one ounce of powder weighs 440 grains. my powder load is 7/440th's of an ounce. that is not much, and it must be weighed and measured, ... , carefully.
why a 150 grain bullet, when 110 to 120 grain bullets seem to give the best performance?
well, if i am looking to shoot a bullet at subsonic speeds, (to avoid a supersonic "boom" or "crack" which would make a shooting position detectable) i am limited to about 1050 fps. so, great amounts of energy are not gonna be created by great amounts of velocity, by definition: i cannot exceed 1050 fps or so and remain "stealthy." so, the only way to increase bullet energy on target is to increase the weight of the bullet, and thereby gaining the added benefit of having a bullet which "flies" better because of greater ballistic coefficient due to its added weight in relation to its frontal area.
to make a long story short, i loaded the 150 grain sierra matchking in front of 7 grains of imrtrail boss powder, and took it to the range for function testing. that is, specifically, a 6.8mm remington spc, using remington cases, cci large rifle standard primers, 7 grains by measured weight of imr trail boss, and a 150 grain sierra matchking bullet, bthp, of .277" in diameter. kapish?
all went swimmingly. there are absolutely no signs of pressure in the fired cartridges, there being a very minimal case expansion in front of the extractor grove of about .001", and normal appearing primers, and absolutely no evidence of partial ignition and/or burning of powder.
and, the "sonic report", or muzzle blast if you prefer, of the imr trail boss was noticeably lighter than that of the accurate arms 5744 powder, or the imr sr-4759. i fired them without ear protection, 3 rounds, to get a valid comparison. and, as an added bonus, the imr trail boss gave complete combustion, there being no residue of unburned powder granules left in the fired cases. i am pretty happy about that as well, and since the imr trail boss fills the case to the bullet, the position of a light powder charge not being reached by the flame of the primer is not a concern either.
wonderful. g_d & hodgdon smiled benignly upon us fools who dabble in these things. i may yet die with all my fingers.
the only bad thing, was that while the imr sr-4759 shot right to the point of aim set up for my normal loads the imr trail boss does not. i will work on that.
i am not yet just exactly where i want to be, but i am getting there, and refining the process.
we'll just work on that aspect of things.
yes, i believe that this is the way to go to get "whisper" capacity out of the 6.8mm remington spc upper without having to custom design a round based on a smaller pistol case and using the .277 heavy bullets in a separate upper or an altogether separate firearm.
i have lower muzzle blast, pretty good accuracy so far, and absolutely no suggestion of any kinds of undue pressures associated with the combustion of the imr trail boss powder in a high powered rifle case. so far, the hodgdon engineers have proved more clever than this particular idiot: it is nice to know that they have taken my proclivities into consideration, and have striven to protect me. i am conferred a special providence, by g_d and hodgdon, and i appreciate both.
to put the matter mildly, i am very happy with this development. all i need do now is get a chronograph, and winnow the velocity down to sub sonic levels, and to develop an accurate load. so far, so good.
i will keep you posted.
john jay @ 11.10.2010
p.s. previous installments on my adventures learning how to get the 6.8mm remington spc to work in the ar-15 upper may be found, here.
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