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9/19/2017 7:27:10 PM
Posted: 10/10/2003 12:42:44 PM EDT
What makes them more accurate? Seems to me that, since most gas-operated semiautos are basically bolt-actions with a few extra parts to manipulate the bolt with a linear motion, that they should have about the same accuracy. So are bolt-action rifles more accurate, and why?
Link Posted: 10/10/2003 12:53:34 PM EDT
A bolt that moves as a result of recoil or gas pressure will have a certain amount of "wiggle" to it, even when in battery. This makes the bolt face less square in proportion to the round, which might affect accuracy.

Hell, I'd guess that even the different "pulse" in the gas pressure that a gas port makes would affect accuracy to some degree.

There are a number of things that'll do it, and I'm sure an expert will be along shortly to splain it all.
Link Posted: 10/10/2003 1:50:20 PM EDT
[Last Edit: 10/10/2003 1:51:18 PM EDT by HoGKiLLa]
mainly the lock up of the action is much tighter and the chamber is usually alot tighter as well compared to semi's. Such as an M16 compared to a .223 M77. The M16 needs to be able to fire alot of different rounds(brands, loads, 556 and 223) and the M77 is for accuracy and will most likely have a hard time feeding and ejecting 556.Especially cheap stuff because the casing will swell in the chamber and stick to where you cant extract the spent casing. Everything on a bolt action is generally tighter, resulting in improved accuracy, since the bolt and bullet are locked up in almost the exact same position every time.
Jeremy
Link Posted: 10/10/2003 2:42:51 PM EDT
Hmm... I suppose you could say that most of the semiautos that I have seen might not lock up as solidly, since the bolt in a bolt-action is usually long and solidly fixed to a linear track while semiauto bolts seem to be held in place mostly by multiple locking lugs. But that still isn't a fundamental reason, really. I don't see any reason why you couldn't design a semiauto with a bolt that locks up just as well as any bolt-action.

The chamber thing is probably right, but that doesn't seem to be a bolt-action versus semiauto thing as much as a tight versus loose chamber thing (or even ammo quality). Yeah, the usual M16/AR15 would be built with slightly looser chambers (5.56NATO) so as to be reliable with more types of ammo and less precisely manufactured ammo, and they would be willing to sacrafice a little accuracy for this, since you don't really need 1/4MOA in a combat weapon (at least, not one designed for close to medium range combat rather then sniper duty). But aren't there also semiautos with tight chambers (.223REM) that lose some reliability for accuracy? Like for a shooting match where all the ammo is handloaded and checked with micrometers, and a failure isn't that big a deal, but really good accuracy is important.

I kinda doubt that the gas port hurts accuracy, since it's exactly the same every time. From my understanding of the subject, accuracy doesn't just depend on consistency, accuracy is consistency. So as long as the gas port is configured the same and the gas powers the same bolt carrier against the same spring, it should be fine.
Link Posted: 10/10/2003 3:08:01 PM EDT
PArt of it also is how the bullet is seated each time. A bolt action's round is seated in the chamber the same way, each time. A semi...well, it can vary on the gas pulse, how dirty the rifle is, how the round was stripped from the magazine/which side of the magazine it was stripped from, etc.

Another point was the bolt lugs..or lack thereof. Most modern rifles didn't lock the bolt to the chamber with locking lugs until fairly recently. An AK's bolt locks to the receiver/trunion. A FAL's bolt locks to it's locking shoulder. So on, so forth. This radically changed with the M16 system, with 8 locking lugs right at the barrel extension. (Well, 7, if you don't count the extractor.) More locking lugs means less vertical or horiziontal movement of the bolt when it's in battery. Bolt guns generally had lugs of some kind that would securely lock the bolt to the barrel. FAR more securely than a semi.

There are other factors (gas pulse as well is one of them), but those are probably the two main ones. A lot of careful tuning can overcome those problems (as in Match M1 Garands, and Match M14's), but you'll never really eliminate them.
Link Posted: 10/12/2003 1:47:57 PM EDT
On a semi auto not only does the bolt seat differently each time but it could potential be unlocking at a slightly different time. Plus with a gas operated weapon the amount of gas being diverted to operate the action can vary some which can affect the MV. If the gas system uses a piston then not only do you have a problem with the bullet not seating the same way after every shot but you can have the piston seating differtly after each shot which can cause pressures to vary.
Link Posted: 10/12/2003 4:32:13 PM EDT
Not to mention that bolt guns are usually fired using brass that was fireformed and necksized specifically for that particular rifle. Semis almost always utilize full-length sized brass that doesn't fit the chamber as tightly to ensure feeding.
Link Posted: 10/13/2003 6:23:35 PM EDT
Some of the Camp Perry shooters tell me that the bolts no longer hold that advantage. I believe them.
Link Posted: 10/13/2003 10:59:42 PM EDT
its a crap shoot..

i have 2 semi autos that shoot sub 1 MOA..
1 a .308 bar
2 a 30-06 remington 742.

never had a jam in either. absolutely wicked beasts.
Link Posted: 10/14/2003 6:15:29 AM EDT

Originally Posted By BobCole:
Some of the Camp Perry shooters tell me that the bolts no longer hold that advantage. I believe them.

...........................................It`s more of a falacy than fact.....everything relates to fit and tolerance.....most bolt guns are made to be accurate to a degree...due to the specific purpose.....semi`s are generally built for a diferent purpose....therefore reducing the accuracy accordingly.....BUT...autoloaders that are built for competition...like match grade....surpass many off the shelf bolts.....as in...."you get what you pay for".....
Link Posted: 10/15/2003 8:48:27 PM EDT
Browning BARS are just as accurate as a bolt action rifle in my experience. My 7mm rem mag BAR shoots sub MOA off of a bench. It is true though the the BAR is the highest quality semi auto hunting rifle you can get. Besides for most shooting purposes(aka hunting,deffense,non competion target shooting and plinking, ect.) the difference in the accracy beteween the two isn't enough to matter much. There for it is my opinion that for most practical purposes a semi auto is preferable to a bolt. Only in sniping or compition is the maginaly better accracy of a bolt preferable to the speed of a semi auto.
Link Posted: 10/16/2003 7:44:45 AM EDT

Originally Posted By Hoplophile:
On a semi auto not only does the bolt seat differently each time but it could potential be unlocking at a slightly different time. Plus with a gas operated weapon the amount of gas being diverted to operate the action can vary some which can affect the MV. If the gas system uses a piston then not only do you have a problem with the bullet not seating the same way after every shot but you can have the piston seating differtly after each shot which can cause pressures to vary.



Why should the amount of gas vary, though? Doesn't accuracy already require consistent powder loads and consistent ignition to produce consistent pressure and muzzle velocity? If all of these things are the same from shot to shot, then why should the gas pulse vary?

I can see the piston seating having the potential to be different from shot to shot. Didn't the AR15 system use gas pressure directly on the bolt carrier to eliminate this problem, though?


Originally Posted By Evil_Ed:
PArt of it also is how the bullet is seated each time. A bolt action's round is seated in the chamber the same way, each time. A semi...well, it can vary on the gas pulse, how dirty the rifle is, how the round was stripped from the magazine/which side of the magazine it was stripped from, etc.



Don't most bolt-actions feed from small internal double-stack magazines, though? And of course, a bolt action can be given lousy ammo or allowed to get dirty too.


Originally Posted By mylhouse:
Not to mention that bolt guns are usually fired using brass that was fireformed and necksized specifically for that particular rifle. Semis almost always utilize full-length sized brass that doesn't fit the chamber as tightly to ensure feeding.



Could you expand on this a little? One of the other things I was wondering is how they make necked brass. I figured you could make the rim and primer area and the wall thickness taper by indenting, but I'm not sure about the neck.
Link Posted: 10/17/2003 1:19:46 PM EDT
A lot of it has to do with barrel harmonics.
The barrel should be able to float freely, hense free floating the barrel.
Semi-autos have a gas block, front sight or whatever on the barrel.
This affects the harmonics.

Bolt guns, since they have fewer moving parts,
generally have better trigger pulls.
Link Posted: 10/17/2003 8:12:10 PM EDT

Originally Posted By mylhouse:
Not to mention that bolt guns are usually fired using brass that was fireformed and necksized specifically for that particular rifle. Semis almost always utilize full-length sized brass that doesn't fit the chamber as tightly to ensure feeding.



Could you expand on this a little? One of the other things I was wondering is how they make necked brass. I figured you could make the rim and primer area and the wall thickness taper by indenting, but I'm not sure about the neck.


mace,

I was referring to reloading ammo. All brass for a given caliber is generally the same and made the same way. I was mainly referring to when you resize a case (because it expands in the chamber during firing). Neck sizing only affects the neck (which holds the bullet in place by tension when it is sized properly), while leaving the rest of the case "expanded". This allows for an almost perfect, tight fit of the reloaded case into the chamber, aligning the bullet more perfectly in the bore.
Link Posted: 10/18/2003 6:39:01 PM EDT
Scottfn308 brought up a good point: barrel harmonics. Excluding accurized semis, the average semi's barrel isn't free floating, and stock bedding/ handguard mounting will affect accuracy.
Link Posted: 10/18/2003 7:38:20 PM EDT
BAR's are free floated.
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