|
|
Post by wfa on Jun 23, 2015 6:56:41 GMT -8
Every barrel manufacturer seems to have their own "ideal port diameter"; "under gassed" is easy to determine; "over gassed" is more subjective until it becomes destructively excessive. My question is "How do you know when it's 'just right'?
|
|
|
|
Post by GLSHOOTER on Jun 23, 2015 8:41:33 GMT -8
Every barrel manufacturer seems to have their own "ideal port diameter"; "under gassed" is easy to determine; "over gassed" is more subjective until it becomes destructively excessive. My question is "How do you know when it's 'just right'? Well to be honest it is rarely "just right." Qualifying that is based on the vagaries of gun powder, pressure, expansion ratio and tolerances. Given a 6MM bore the expansion ratio will be bigger than a 223. That is the amount of space that XXX powder being burned has to expand into. A linear 16 inches of 223 bore has a volume of approximately 0.6249 cubic inches. A 243 of 16” will have about 0.7481 cubic inches. That means the 6 MM has approximately 19.7% greater volume. Same powder charges as in 223 vs. 6X45 will be lower in pressure at the port because of the increased bore. Lower pressure means less gas going through the gas port. That means we are dealing with less “push”. The port should ideally be bigger in the 6X45 but in reality most makers will just drill a slightly oversized port that will over gas some, be just right for others and under gas others. Expansion ratio is just one aspect. Different powders produce different amounts of gas due to burn rates and charge weights at acceptable pressures. Another balancing act for the reloader to handle. How that pressure is running at the gas port can be crucial for consistency and accuracy. Tolerances on the rifle. How slick is your BCG? Do you have an m16 or AR15 BCG? How heavy are your springs? Is the port lined up for 100% gas flow? On and on. Heavy buffers, poor port alignment, and heavy BCG’s all require more gas to run. All the above come into play. Now to get it distilled down to, “just right.” You are looking for a couple of factors. Ejection every time that does not throw it a country mile or drop it at your feet, in most cases, and has enough oomph to lock the bolt back on an empty magazine. You are looking at the case heads for marking. On KNOWN good, normal, pressure loads you should not be seeing any ejector “swipes” or heavy extractor imprints in the case head or bent rims. Reloading the next cartridge should be consistent with no over-riding of the round in the magazine. How do we handle getting to the “just right” can be through powder selection to get the pressures curve right or we can cheat and run a slightly bigger port with an adjustable gas block. The ability to tune the flow is a major help. We’ve all seen the charts, easily found, that show under/over gas activity. I tune mine accordingly. I will over gas on purpose if I absolutely have to have 100% reliability 100% of the time. Most of the BHW barrels have a more than generous gas port and can be run successfully with a standard non-adjustable GB. I want the ability to work with a wide selection of powders. I’ve not had to drill a port yet and I have loaded a huge cross section of powders in various bore sizes and run an adjustable on 90% of my rifles. Belts and suspenders are never bad to have. Just some thoughts on this one. I know I’ve missed a huge amount of discussion points but that’s my starter talking points as they say. Greg |
|
|
|
Post by wfa on Jun 23, 2015 10:04:56 GMT -8
Thank you Greg for you thoughts, facts, and opinions. Now I am educated enough to refine my question.
I have three 300 AAC BLKs that I play with; each has a different diameter gas port, and each, with one specific load exception, wants a different load for accuracy and function (both super and sub).
Just as with people, I realize that each rifle is "an individual", but are there are certain things that will keep them "happy", and most importantly, "what are the signs of happiness"? People and rifles seem to both love money. What else will "put a smile on their face"? What would be considered ideal ejection "distance", ideal location of the brass pile, ideal arc height of ejection (I've been next to, many times, shooters with AR's which seem to love to throw brass at my head when standing, down my neck when prone!). I realize there are probably no "etched in stone" answers, but opinions and "WAG"s do count!
|
|
|
|
Post by GLSHOOTER on Jun 23, 2015 11:06:50 GMT -8
Thank you Greg for you thoughts, facts, and opinions. Now I am educated enough to refine my question. I have three 300 AAC BLKs that I play with; each has a different diameter gas port, and each, with one specific load exception, wants a different load for accuracy and function (both super and sub). Just as with people, I realize that each rifle is "an individual", but are there are certain things that will keep them "happy", and most importantly, "what are the signs of happiness"? People and rifles seem to both love money. What else will "put a smile on their face"? What would be considered ideal ejection "distance", ideal location of the brass pile, ideal arc height of ejection (I've been next to, many times, shooters with AR's which seem to love to throw brass at my head when standing, down my neck when prone!). I realize there are probably no "etched in stone" answers, but opinions and "WAG"s do count! For me the ideal will be 4-5 feet. I try to tune mine closer to 4:00 as opposed to 3:00. I'm a lefty and I don't find it objectionable even with a slick side. The brass deflector buys me space. Here is a link to a well done description: img.photobucket.com/albums/v503/katwoman214/brian/arejectionpattern.jpgWhen I get down to playing hard I just want it out of the gun. I'll run a bit more gas if I can and that way I'm not ever on the ragged edge of a lighter load getting me caught with a jam. I have never had a BHW be undergassed even with a standard GB. Like most barrel makers better a little more than a little less though some have had to lightly open up the port. Many of those same guys though want to run heavy buffers and springs. I NEVER do so it has been moot for me. Why induce problems when you don't have too? Greg |
|
|
|
Post by wfa on Jun 23, 2015 12:24:15 GMT -8
That picture is worth the proverbial 1000 words!
I don't like heavy springs or heavy buffers either - my lowers (with the good triggers) get switched too often.
|
|
|
|
Post by GLSHOOTER on Jun 23, 2015 14:10:03 GMT -8
That picture is worth the proverbial 1000 words! I don't like heavy springs or heavy buffers either - my lowers (with the good triggers) get switched too often. And surprisingly it's accurate.  Greg |
|
|
|
Post by dirtysteve on Sept 5, 2015 19:19:07 GMT -8
I know this is an older thread but just going to toss out there that adjustable gas blocks are the way to best fix any over gassing. My wives LAR-8 was wickedly over gassed and was hell on brass. After I installed an adjustable block it shoots better and my brass doesn't take a beating.
|
|
|
|
Post by GLSHOOTER on Sept 5, 2015 19:50:48 GMT -8
I know this is an older thread but just going to toss out there that adjustable gas blocks are the way to best fix any over gassing. My wives LAR-8 was wickedly over gassed and was hell on brass. After I installed an adjustable block it shoots better and my brass doesn't take a beating. They help a great deal as I mentioned back in June. 90% of the uppers Ritch and I shoot are adjustable. Greg |
|
|
|
Post by ivlianvs on Nov 1, 2015 12:31:31 GMT -8
I know this is an older thread but just going to toss out there that adjustable gas blocks are the way to best fix any over gassing. My wives LAR-8 was wickedly over gassed and was hell on brass. After I installed an adjustable block it shoots better and my brass doesn't take a beating. Popping in even later to this old thread. You are preaching to the choir about that LAR-8 problem. Sheesh! I get the reliability concern, but that thing destroyed almost 100 rounds of my best brass before I realized that it was happening! I used to work with pneumatic systems (multiport valves, piston actuators, etc.), and I used to be opposed adjustable blocks as a sales gimmick and a band-aid solution to a poorly designed system. Greg touched on some of the principles of pneumatic force back in June, and sparing a college level lecture on the physics of pneumatic forces and the design of the AR (although the SCAR is much worse), I will simply say that I have since revised that opinion and don't build a rifle without an adjustable gas block. I only have one upper that isn't sporting a Syrac ordnance Gen 2. I love the click adjustments on them that stays in place. If trying to squeeze out a little more accuracy without trying to race a clock, I prefer a heavier buffer and bolt carrier systems for increased lock time. This requires more gas, typically obtained by a higher flow rate, to get all that extra weight moving. I have even added a Tubbs carrier weight to my own LAR-8. However, everything has it's trade offs. More moving mass makes the rifle rock more as the parts are moving around. That LAR-8 rocks like a small boat on the high seas. So, if you're racing a clock such as in a gunfight or on the action format competition scenes like 3 gun, it slows down followup shots as it takes longer for the movements to settle down. I do run more "low mass" carriers than full weight, and standard over heavy buffers. My opinion on the barrel porting is that it should be opened as wide as possible, within limitations of the bore diameter. Going on the principle of messing with the most easily replaceable component, the barrel is the most expensive part of almost every rifle. Do the porting once with room to spare, and let the cheaper more easily replaceable gas block be the piece that restricts the flow, whether adjustable or not. I would rather have to buy another block for under $100 than a barrel for $400 and up. Also, I can drill out an aluminum gas block on my press. I'm not touching a barrel on my own. I might be able to drill the steel with the previous gas port already there as a pilot, but cleaning up that exit wound inside the bore is more than I want to mess with. Just another reason to appreciate BHW barrels, as Greg tells us they have a more generous port. |
|
