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Guys, from a 22-inch barrel, .308 Win chambered M-1A shooting 175 gr. Sierra Matchkings over 44.0 gr. of Varget (2,620 fps MV from 20-inch Rem 700 barrel), how much velocity should I expect from the M-1A? Relative to the bolt gun, should I expect to lose 150 fps in the M-1A due to gas system inefficiencies?
 

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Someone with much more knowledge than me can pipe in here if I'm off base, but I don't think you should see too much of a difference in MV out of a comparable length bolt action. The bullet should have left the muzzle before there would be an appreciable amount of gas in gas system, so the gas system itself shouldn't be the reason for any velocity loss. What you need to look out for is port pressure, and that's the reason why recommended velocities are generally lower than bolt guns.
 

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Gents,

I get almost identical velocities between Celia Phayte (my M-14) and the 22" barrel on my G.A. Precision "Rock". Any differences are due to the individual rifle's characteristics and not the load. Hope this helps.
Recommend that you chrono both just to see what the differences are...

Wes
 

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Why would you expect a loss of velocity, you are moving from a shorter barrel to a longer barrel and the gas system isn't so inefficient that you loose velocity. My guess is that you'll see an INCREASE of around 50 FPS.
 
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RAMMAC I don't think all 22" on the M1A is rifled. I'm thinking a muzzle device is stealing some length. I'd say about an 1" different between the two. Gas bleeding from the M1A would be the velocity thief.

How much loss? I'd think it's too close to call.
 

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You are correct that the whole 22" is not rifled, there is the chamber and the cutaway for the bolt head, other than that it is rifled all the way to the end except for the crown, but the muzzle device length is in addition to the 22" barrel.
m14brian
 

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RAMMAC I don't think all 22" on the M1A is rifled. I'm thinking a muzzle device is stealing some length. I'd say about an 1" different between the two. Gas bleeding from the M1A would be the velocity thief.

How much loss? I'd think it's too close to call.
What gas bleeding? The amount of gas used by the system is minor and of no consequence.

Compare apples to apples, all 20 inches on the bolt gun isn't rifled either. Barrel lengths are measured from the chamber to the muzzle. In a 20 inch barrel the rifled part is about 18 inches, in a 22 inch barrel it's about 20 inches, that's understood by anybody that works with firearms, the bottom line is that the bullet will have about 2 more inches to build velocity.

I've been building handloads for several decades and I'll guarentee that there will be an increase in velocity.
 

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I agree , the 22" BBL. should have a slight increase in velocity , the only other thing (s) that could alter velocities is the Barrels Rifling & how its made , I have read 5R rifling can increase velocities , but I defer that to someone who has a 5R Barrel .
 

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From a document entitled:
U.S. Army Weapons Command Research & Engineering Directorate Small Arms Testing Laboratory
Technical Notes
Small Arms Weapons Design
John G. Rocha
May 1968
Page 67

In considering the utilization of gases vs. the effect upon muzzle velocity, there is only an insignificant loss of projectile velocity, averaging about .5%.
At the velocities we are talking about that's a worst case scenario of 13 FPS lost to the gas system. Further, when they claim an average velocity loss of .5% they come to that conclusion after testing all types of gas cutoff systems with a very wide variety of designs. The M14 actually has less velocity loss than .5%.

My QuickLoad software estimates that there would somewhere around a 50 FPS increase based on barrel length. So at worst you'd see around 35 FPS faster velocities in the 22" barrel.
 
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Man all y'alls been cooped up too long. I just pointed everything out for this questioned test.

RAMMAC - What gas bleeding? The M1A does vent gas to cycle the action so there is a loss there in velocity.

20" barrel on a 22" M1A barrel? Kind a wonder what muzzle treatment is on that measured barrel. Them flash suppressor ain't 2" or 3" so your math that 20" barrel don't jive.
 

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Man all y'alls been cooped up too long. I just pointed everything out for this questioned test.

RAMMAC - What gas bleeding? The M1A does vent gas to cycle the action so there is a loss there in velocity. [1]

20" barrel on a 22" M1A barrel? Kind a wonder what muzzle treatment is on that measured barrel. Them flash suppressor ain't 2" or 3" so your math that 20" barrel don't jive. [2]
1. Actually, in the M14 it does not vent until well after the bullet leaves the barrel. When the gas enters the gas port, it still is in a closed volume and therefore continues to provide pressure to the system. Further, the small size of the gas port throttles the gas flow limiting the total mass that can escape. In the case of the M14, the pressure in the gas piston only rises to 9,500 psi by the time the bullet exits the muzzle. The total pressure loss due to the gas port is in the order of less than a tenth of a percent.

2. Barrel length, from an interior ballistic point of view, is measured from the base of the bullet as chambered to the muzzle, because that is the distance the gas gets to act on the bullet. The M14 "barrel length" of 22 inches includes the chamber of 2 inches. So, the actual working length of an M14 barrel is 20.211 inches long. (For M80 Ball, note the length is to the base of the bullet, so the deeper the bullet is seated in the case, the longer the working barrel length.)

. . . an average velocity loss of .5% . . .
M852 Match and M118 ammunition is specified to a velocity of 2,550 fps. Half a percent of that is 12.75 fps.

The allowable variation in muzzle velocity for these cartridges is plus of minus 30 fps for a total range of 60 fps, with a standard deviation of 28 fps.

Any velocity change due to the gas port will be lost in the "noise" of the ammunition velocity variation.
 

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A little more about the "noise" . . .

Let's say we have really good lot of match ammunition with an average velocity of exactly 2,550 fps and a standard deviation of 14 fps (half of the specified value).

That means that only 68% of all rounds will have a velocity between 2,536 and 2,564 fps, and 95% of all rounds will have a velocity between 2,522 and 2,578. And, we are trying to measure a drop from 2,550 fps to 2,537 fps.

Assuming two identical barrels differing only by a gas system that robs 0.5% of the velocity, in order to measure this drop, with the above ammunition with a 50% confidence level (50-50 chance the results will be accurate), you would have to shoot somewhere in the 250 to 500 rounds. And, that is with a 100 percent accurate chronograph. If the chronograph has a 0.5% round-to-round variation, which is typical of laboratory grade chronographs (consumer grade versions being closer to 1% or more) the number of rounds required quadruples.

If you start to throw in variables like throat condition, surface roughness, crown condition, leade diameter and length, the confidence level drops quickly. Sort of like predicting the out come of an election based on asking ten or twelve random people, you might get the correct answer, but that would be mostly a matter of luck.
 

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To All:
I have actually shot this load thru my M1A. The components are as follows: Federal Brass, 44.0/ Varget, Rem 9 1/2 primer and a 175 Sierra. Chronograph data for 5 shots 2664,2666,2661,2663,2659. ES was 7 SD was 2 and average velocity was 2662 fps. Going by the original posting of 2620 fps it would seem 2" of barrel added 42 fps, this is right in line with what RAMMAC said.

John
 

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Man all y'alls been cooped up too long. I just pointed everything out for this questioned test.

RAMMAC - What gas bleeding? The M1A does vent gas to cycle the action so there is a loss there in velocity.

20" barrel on a 22" M1A barrel? Kind a wonder what muzzle treatment is on that measured barrel. Them flash suppressor ain't 2" or 3" so your math that 20" barrel don't jive.
Obviously I shouldn't assume that everyone would recognize that I was being a little facetious.

Yes there might be some loss of gas due to poor fit of parts but as was explained by lysander, the gas that is ported actually continues to pressurize the space behind the bullet, the only difference is that the volume behind the bullet is increased very slightly.

I think that your unaware that due to the speed of the bullet and the mass of the gas system parts, the piston doesn't even begin to move until the bullet has left the barrel. The volume that the gas is "bled" to is so small that the effect on the bullet's velocity is of no importance compared to other losses in the system. It's a proven fact that the friction between the bullet and the lands of the barrel has a greater effect on the velocity.
 

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Guys, from a 22-inch barrel, .308 Win chambered M-1A shooting 175 gr. Sierra Matchkings over 44.0 gr. of Varget (2,620 fps MV from 20-inch Rem 700 barrel), how much velocity should I expect from the M-1A? Relative to the bolt gun, should I expect to lose 150 fps in the M-1A due to gas system inefficiencies?
40 ish fps.
 

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I knew it , its all about the math , I should have paid more attention in school , ugh wait , I'm a drop out , I didn't pay attention to any of it .😜
 

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That's the thing I like about loading your own ammo, yes, it can be about detail and math or it can also just be a canned process that produces ammo that is reliable and in general hits to your satisfaction.

I build precision ammo for the rifles that can take advantage of it. My problem is that I enjoy knowing why things work the way they do. My current shooting book is the third edition of Ballistics by Carlucci and Jacobson, I read the first edition years ago. I have several books on ballistics that are authored by Bryan Litz. My oldest ballistics book is Principles and Practice of Loading Ammunition by Naramore, published in 1954, I've read it so many times that even the bookmarks are worn to the point of being difficult to read.

Theory is great and I believe in pointing out the truth of the theory at all times but you have to balance it against experience too. When the two don't seem to agree then I ask why and try to find out through research and experimentation. Most hand loaders don't care about the kind of detailed information that I know and if they follow the general rules about safety then they usually have no problem with getting the coarse results that they are looking for. But what I object to is the guy that doesn't understand the science and then tells everybody that it's unimportant, that's dangerous and it's why we read so many things on the Internet that are just plain wrong.

If you like simple then that's great, but that doesn't qualify you to know how things really work, your only expertise is what has worked for you and that's all you can really share. And that's important, without real world testing and sharing those experiences you'd never have guys like me asking why things happen and then the science would never go forward.
 
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