Do Individual Firearms Prefer Specific Ammo?

[Grayling14]

I have read a number of blogs where the authors mentioned that when zeroing a firearm the operator should experiment with different loads and bullet weights to determine which one " that particular gun likes", and therefore is most accurate with.

The implication is that my 5.56 with a 1:9 twist and 18.50 inch barrel may be most accurate firing a 55 gr. bullet, whereas my buddy's identical gun, (same make, model, rifling, barrel, etc.), may be most accurate firing a 62 gr. bullet.

Does this make sense to anybody, because I'm not gettin' it?

[John_Wayne777]

Most firearms will exhibit their best accuracy with a specific loading due to a whole host of different factors. It is entirely possible to have two rifles built in the same plant side by side that have a preference for very different loadings.

[Complication]

Most firearms will exhibit their best accuracy with a specific loading due to a whole host of different factors. It is entirely possible to have two rifles built in the same plant side by side that have a preference for very different loadings.

I know "a whole host of different factors" was intentionally broad, and the real answer might be horrifyingly complex, but why?

I accept that this is true because I've heard it from a lot of people whom I believe know what they're talking about. But at the same time, it strikes me as a little bit of black magic.

I'm with you that two identical rifles will fire the same exact loads differently (perhaps one more accurate than the other) but I'm struggling with how those identical rifles might 'prefer' significantly different loads. Someone walk me (slowly) from here to there, if possible.

[montanadave]

I'm by no means well versed in this area, but I can offer a layman's explanation based on what I have been told. Manufacturers produce parts to specifications which allow for variations within an acceptable range (tolerances). So each component part will likely be a little "plus" or a little "minus" of the exact specification for that part. And when you start putting together a significant number of parts, these tolerances are compounded in the finished product. On the whole, most completed products will have about as many "plus" parts as "minus" parts so it's a wash. However, there will be a given number of completed products which have more "pluses" than "minuses" (and vice versa) and this will result in a slightly different result, despite the fact that all component parts were manufactured to specification.

End result, manufactured products comprised of a multitude of different component pieces will often times have a slightly different "personality."

[Complication]

Alright, but it seems to me that the bullet interacts with a relatively minor subset of all of those parts.

Yes, each cartridge has "pluses" or "minuses" in its specifications, but if the idea is that one load will consistently out-perform a different load whereas the opposite is true in an identical rifle, then the variations between different individual cartridges would be negligible in this discussion.

SO...

You've all these variable parts but none of them matter in the time BEFORE the primer is struck. Now, once the primer is struck, you've got the cartridge fired and the casing expands against the chamber walls (item 1 in our variable parts list), then the bullet flies down the barrel (item 2). It passes the gas port (item 3) where some gas bleeds off, and eventually exits the barrel past the flash suppressor (item 4--although it may not physically touch the bullet, it affects barrel harmonics [with variability] and gas expansion [also with some variability]).

Are those 4 items enough to significantly effect bullet performance with regards to the discussion? Or is there more to it?

[montanadave]

My short answer is yes, but I'm guessing some folks may want a more well-defined interpretation of what you consider "significant."

As previously noted, my comment was based on a layman's understanding. I'm going to butt out and allow those forum members with far more expertise on this subject to continue the discussion so I can learn as well.

[xfyrfiter]

Rifling twist can vary from barrel to barrel even from the same mfg. My 1/9 twist bbl. has been measured( twice to just be sure) to be much closer to 1/8, than stated 1/9. Not necessarily a bad thing, just manufacturing differences.

[High Desert]

If you enjoy reading, the following may help. Rifle building for accuracy is nothing new under the sun and many things learned 100 years ago still apply today.

The Muzzle Loading Cap Lock Rifle by Maj. Ned Roberts

The Bullets Flight from Powder to Target by Dr. Franklin Mann

Don’t laugh, these are phenomenal texts.

And yes, most rifles have a personality.. That’s why we name them. ;)


HD

[TY44934]

You correctly ID'd the issue of +/- tolerances as being part of this phenomenon. Extend that to THE main part which the bullet interacts with: the barrel.

Barrels start life either as a blank bored on a "gun drill" (google it) or hammer forged from a short blank.

When new, the drill or the mandrel are at the extreme "+" side of the tolerance.

Now, say that drill or mandrel is used 500 times. It will wear. It will be replaced as it nears the extreme "-" end of the tolerance.

Are ALL 500 barrels "within +/- specification" ?? Of course.

Are all 500 identical? No. Most likely, the widest variation would be between barrel #1 and Barel # 500.

Obviously, these two barrels would NOT perform in exactly the same way.

In short: tool wear is an issue.


Alright, but it seems to me that the bullet interacts with a relatively minor subset of all of those parts.

Yes, each cartridge has "pluses" or "minuses" in its specifications, but if the idea is that one load will consistently out-perform a different load whereas the opposite is true in an identical rifle, then the variations between different individual cartridges would be negligible in this discussion.

SO...

You've all these variable parts but none of them matter in the time BEFORE the primer is struck. Now, once the primer is struck, you've got the cartridge fired and the casing expands against the chamber walls (item 1 in our variable parts list), then the bullet flies down the barrel (item 2). It passes the gas port (item 3) where some gas bleeds off, and eventually exits the barrel past the flash suppressor (item 4--although it may not physically touch the bullet, it affects barrel harmonics [with variability] and gas expansion [also with some variability]).

Are those 4 items enough to significantly effect bullet performance with regards to the discussion? Or is there more to it?

[CAVDOC]

also consider the feed cycle- a round on its way to the chamber can be affected by small irregularities in feed ramps chambers and what not,and was the barrel in your rifle the last one done before replacing some portion of the tooling or right after- once you start using the tooling it is estimated it will last for "x" number of cycles of use but with each cycle a little wear accumulates on the tools- while barrel #1 and barrel #999 off the same tool will be within spec the measurements of each may be quite different.
in closing it is not improtant in this case to understand why just recognize it exists

[Pathfinder Ops]

I'll just add this and it isn't a scientific study and not an AR specific observation but:

My Ruger 10/22 that I have had for friggin ever and is a stone cold tack driver does not run anything as well as it does the standard issue, over the counter Federal JHP's. The stuff you buy in a brick from Walmart.

Any other manufacturer or better quality stuff and suddenly I'm getting everything from failure to feed to failure to eject to a decrease in accuracy under the same shooting conditions.

I don't know why and I am no armorer or ballistics expert. That's just my street (range) experience.

That being said i can say the I have shot 3 different manufacturer's .223 through my AR and I do not sense any difference in the performance of the gun or my accuracy.

I'm inclined to think that there are ideal loads for certain guns... why? I don't know. Magic maybe..........;)

[CarlosDJackal]

I know "a whole host of different factors" was intentionally broad, and the real answer might be horrifyingly complex, but why?

I accept that this is true because I've heard it from a lot of people whom I believe know what they're talking about. But at the same time, it strikes me as a little bit of black magic.

I'm with you that two identical rifles will fire the same exact loads differently (perhaps one more accurate than the other) but I'm struggling with how those identical rifles might 'prefer' significantly different loads. Someone walk me (slowly) from here to there, if possible.

It's pretty simple, really. It's all about the Physics. Harmonics are very unpredictable. Why do you think that musical instruments that are made by the same person using the same materials and techniques from the same source at the same time can still end up being different?

No black magic involved. Even when made from the same batch of steel at virtually the same time two barrels may have enough of a difference at the molecular level to have different harmonics once they are used.

One of the factors that define the accuracy of a rifle/ammo combination is where the muzzle is in when the bullet exits. A barrel will "whip" when hit with a hammer or when a cartridge is ignited. If a projectile leaves that barrel at the close to the same point of its "whipping" action, the you may very well have an accurate match.

There is also the possibility that the barrels' twist is not exact. Some 1:9-inch twist ARs loves 75-grain bullets because they are closer to a 1:8.5-inch twist. While some, even from the same lot, may only prefer a maximum bullet weight of 69-grains.

One of these factors is how sharp a cutting tools stays between barrels. There is bound to be some loss of sharpness with each use and this may be enough to make the difference.

Of course, I could be full of crap and someone will eventually come along to correct me (it wouldn't be the first time). :D

[Complication]

It's pretty simple, really. It's all about the Physics. Harmonics are very unpredictable. Why do you think that musical instruments that are made by the same person using the same materials and techniques from the same source at the same time can still end up being different?

No black magic involved. Even when made from the same batch of steel at virtually the same time two barrels may have enough of a difference at the molecular level to have different harmonics once they are used.

One of the factors that define the accuracy of a rifle/ammo combination is where the muzzle is in when the bullet exits. A barrel will "whip" when hit with a hammer or when a cartridge is ignited. If a projectile leaves that barrel at the close to the same point of its "whipping" action, the you may very well have an accurate match.

There is also the possibility that the barrels' twist is not exact. Some 1:9-inch twist ARs loves 75-grain bullets because they are closer to a 1:8.5-inch twist. While some, even from the same lot, may only prefer a maximum bullet weight of 69-grains.

One of these factors is how sharp a cutting tools stays between barrels. There is bound to be some loss of sharpness with each use and this may be enough to make the difference.

Of course, I could be full of crap and someone will eventually come along to correct me (it wouldn't be the first time). :D

Aha! I had forgotten to consider harmonics. Sounds perfectly reasonable now. Thanks.