Semi Auto verse Bolt Action- muzzle velocities

ivanthehunter

Has anybody here ever measured the power loss of a semi auto when compared to a bolt action with the same barrel length and of course the same caliber..



Would it be at all possible to keep the sliding bolt in a semi auto closed for a test shot?....................NB NB--- I would not attempt this in reality as i like my body they way it is at the moment.. (a little weight loss perhaps)


If i did this what would i find when the bullets go through a chronograph when
A: the bolt sliding in auto mode
C: the bolt some how closed and secured


Please fell free to to comment on any caliber but .22lr and .22WMR would be prob most applicable in Ireland and as such they are unrestricted..

murph226

I didnt think there was many rifles that fired from the open bolt position

milkerman

Walther made a semi auto in .22 during the 1930's that was a hybrid of the bolt action. Worked like this, -Bolt forward with toggle down=single shot. Bolt forward with toggle up=semi auto action. There were still a few of these guns floating around in the 80's but havent seen one in years. I believe the Krico also had an option for single shot and manual cycling.
Interesting question but if viewed from a rabbits perspective a bit like how many angels can dance on a needle point!

ivanthehunter

ivanthehunter wrote: »

Would it be at all possible to keep the sliding bolt in a semi auto closed for a test shot?....

Murph -- You know what i was asking...... but here you can chew on this..


Yes the sliding bolt is closed for any shot in a semi but it is only backed by spring pressure and the inertia of the sliding bolt.


This is what I wanted you to consider.
"Closed for a test shot" similar to the fixed closed style of a Bolt action..

ivanthehunter

milkerman wrote: »

Interesting question but if viewed from a rabbits perspective a bit like how many angels can dance on a needle point!

Classic lol
:D

murph226

ivanthehunter wrote: »

Murph -- You know what i was asking...... but here you can chew on this..


Yes the sliding bolt is closed for any shot in a semi but it is only backed by spring pressure and the inertia of the sliding bolt.


This is what I wanted you to consider.
"Closed for a test shot" similar to the fixed closed style of a Bolt action..

Is the bolt not locked in place for the shot by locking lugs?

Jonty

ivanthehunter wrote: »

Would it be at all possible to keep the sliding bolt in a semi auto closed for a test shot?....................NB NB--- I would not attempt this in reality as i like my body they way it is at the moment.. (a little weight loss perhaps)

Krico made a .22 semi that you could lock the bolt on and make it into a "bolt action".

Rew

Jonty wrote: »

Krico made a .22 semi that you could lock the bolt on and make it into a "bolt action".

I have that Krico. You lock the bolt by pushing the cocking handle in. Great rifle, extreamly quiet but it doesnt cycle well.

Vegeta

I am not expert but my guess would be as follows.

For there to be a loss in power then some of the gas caused from burning the propellant must escape, somewhere, before the round leaves the barrel. I don't think that happens with a semi auto otherwise you'd notice it.

The round has left the barrel before the bolt cycles backwards so I cant see why or where you'd lose power.

https://www.youtube.com/watch?v=d4S9f0D0THc

Even with the relatively slow speed of a shotgun load, it has left the barrel before the round gets ejected.

See the second clay being shot in the following video. Watch the end of the barrel carefully

https://www.youtube.com/watch?v=GlGeudHY5Ww

.50 (MOA)

I have the Krico rifle with the dual operation mentioned. The bolt handle simply pushes into a hole in the receiver same as the hole used to lock the bolt open and can easily be used in either mode, you either push the bolt handle in for single shot or leave it out for semi auto.

I found it very innaccurate at in semi auto, but reasonable in bolt action, so i use it mainly in bolt action. its good to have the choice. its like having two guns in one almost.

As for chronographing the bullet speed, i don't have any access to a chronograph to do this. but i did once do a very rough experiment with a piece of wood and found that in single shot it penetrated about 2 inches, and 1.5 inches in semiauto. I know this is nowhere near an accurate experiment and it only shows what was already known- that there is a drop in muzzle velocity, it doen't show how much drop that is.

Vegeta

.50 (MOA) wrote: »

I know this is nowhere near an accurate experiment and it only shows what was already known- that there is a drop in muzzle velocity, it doen't show how much drop that is.

Really didn't know that there was a drop in muzzle velocity when using semi vs bolt and using same ammo and same barrel length.

Hence my above post. Please ignore and carry on teaching me stuff

Sparks

Vegeta wrote: »

The round has left the barrel before the bolt cycles backwards so I cant see why or where you'd lose power.

Because some of the propellant gas is being diverted off away from pushing the bullet to pushing the mechanism that cycles the bolt; so some power has to be lost. Which is why .22 semi's seem to cycle better on supersonics rather that subsonics - there's more energy there to draw from in the first place.

Nonetheless, from the rabbit's point of view, it's all very academic if the first shot is well aimed.

ivanthehunter

murph226 wrote: »

Is the bolt not locked in place for the shot by locking lugs?

No locking lug on a a normal semi auto,

ivanthehunter

Rew and .50(MOA)
Can you submit any photos if they are to hand!! Please:)

Sounds like a great little idea and TBH i dont know why its not on more semi's.

What sort of bearing surfaces are we talking about ie steel on steel and what size are the contact surfaces, also is the bolt action similar to other B/A's in that it is a tight wedged/tapered fit..

The penetration of the timber would suggest a 25% loss in power which is massive.:mad: Not a happy chapy if i'm losing that much power

Also when were these jewels built and whats the model number..

.50 (MOA)

From the theoretical side of things. When the bullet is struck, and the powder begins to burn the pressure inside the barrel increases. This pressure forces the bullet down the barrel, and in a semi auto, this pressure also forces the bolt back to cycle the action. it is the strength of the spring and the weight of the bolt that resists this pressure and means that the bolt does not fly backwards leaving the bullet lodged in the barrel (as would happen if there was no recoil spring) (see the thread about changing a .22lr rifle action to a 17hmr and needing a heavier bolt).

It can be looked at from the point of view, that in a bolt action, the work output from firing a round is a bullet of a certain mass flying at a certain velocity.

In a semi auto there is again the work output of a bullet flying at a certain velocity, but there is also a bolt of some mass moving at a certain speed and a spring being compressed, each of these requiring some form of energy input to cause this movement.

Assuming the bullet weight remains the same, and the energy released when the powder burns is equal. The energy transferred to the bolt and spring removes from the energy that can possibly be transferred to the bullet. This loss in energy (muzzle energy)- provided the bullet weight has remained constant- MUST lead to a reduction in muzzle velocity.

What it comes down to is that for any identical rounds there is a limited amount of chemical energy contained in the powder. A semi auto utilises some of this energy to cycle the action so it must mean that less energy is transferred to the bullet, with the bullet weight remaining equal the velocity must be reduced.

Kinetic energy = half x mass x velocity squared

bullets have kinetic energy when in flight
the mass remains constant (Bullet weight)
the velocity is the only thing that can vary

As a final year mechanical engineering student I would hope to have some understanding of these principles. Combined with my experience of the increased bullet drop and reduced penetration when using my rifle in semi auto. It would lead me to be convinced that there must be a drop in muzzle velocity.

hope this explains the princiles of operation of a semiautomatic to everyone

Vegeta

I understand there would be some loss (more in a gas recoiled semi auto) but I thought it would be a tiny % as to be negligible in the spring operated or inertia driven ones

Obviously its not so tiny

murph226

ivanthehunter wrote: »

No locking lug on a a normal semi auto,

Normal as in spring operated?

Rew

The operation of the likes of an AR15 is deffernat to the operation of my Krico. An AR15 feeds gas back in to the body to operate the mechanism. This happens via a hole in the barrel connected to a tube that runs back to the body.

My .22lr is a blow back bolt that is simply pushed back by the forces of the explosion. Neutons laws and all that (For every action thre is an equal but oppisite reaction).

Not sure about AR's but the likes of a Syter Aug has a rotating lock bolt where the bolt is engaged in the same way as a bot action (lugs and a rotating bolt). While the action dosnt move untill after the bullet has left the barrel the gas has been syphened off while it is still in the barrel. (normally about 1/2 to 2/3 of the way up the barrel)

Sparks

Vegeta wrote: »

I understand there would be some loss (more in a gas recoiled semi auto) but I thought it would be a tiny % as to be negligible in the spring operated or inertia driven ones
Obviously its not so tiny

Well, in theory at least, it'd be the same; you have to siphon out X% of the energy in the system to cycle the bolt, so whether it comes from the gas or from momentum or whatever, it's still not available to push the bullet...

.50 (MOA)

Sparks wrote: »

Well, in theory at least, it'd be the same; you have to siphon out X% of the energy in the system to cycle the bolt, so whether it comes from the gas or from momentum or whatever, it's still not available to push the bullet...

Your right, provided the same energy is needed to cycle the action- it will be different for different guns. the position of the gas tube along the barrel is the most important consideration when the designers are designng these guns.

Too early and the action will be cycled too soon and gas wil be lost out of the breech the bullet could even get stuck in the barrel.

Too late and not enough gas will go into the tube to cycle the action before the rapid drop in pressure when the bullet exits the barrel.

Its also important to note that the longer the tube is from the barrel to the action the greater the energy taken away from the bullet because there is more gas needed to increase the pressure in this tube to the pressure needed to cycle the action.

For an inertia operated semi auto it is the strength of the recoil spring that determines how quick the action cycles.

Too light and it would cycle too quick and the bullet could lodge in the barrel and gas being lost out of the breech.

Too heavy and it wont cycle. similar to the situation with supersonic and subsonic rounds. problems with actions not cycling with subsonic rounds is due to the spring being too strong because it has been designed for supersonic loads.

Jonty

Rew wrote: »

I have that Krico. You lock the bolt by pushing the cocking handle in. Great rifle, extreamly quiet but it doesnt cycle well.

PM sent

bunny shooter

murph226 wrote: »

Is the bolt not locked in place for the shot by locking lugs?

Ah, the Steyr AUG 5.56mm training stands to you :P

Manic Moran

No locking lug on a a normal semi auto

Most any AR-style bolt/breech block, including AUGs, which rotates is locked in by lugs. My AK-like SIG-556 also has lugs, but they're much bigger than the star pattern on an AR.

Even my FAL has what is in effect a locking lug, though they call it a locking shoulder. Performs the exact same function, though, it prevents the bolt from moving to the rear at all until the carrier has been moved clear. The difference is that it is behind the bolt, instead of in front of it.

NTM

murph226

bunny shooter wrote: »

Ah, the Steyr AUG 5.56mm training stands to you :P

yes indeed;)

rrpc

You can even see this effect with a bolt action. I had a thread over on the target shooting forum about the SK ammo which I'd tried out. There was observable differenc on the target between rounds that fitted tight into the breech and those that were loose going in.

The loose ones had a lower point of impact and I had difficulty ejecting the spent shells from these. The way I saw it was that the case expanded to fill the breech when the shot was fired leaving less propellant force to drive the bullet. The expanded case was then harder to eject.

The ones that were tight didn't have any expansion and all the propellant went down the barrel and the shot struck higher on the target.

When you have such differences in tolerance within a batch of ammo, it's advisable to use a headspace gauge to sort the ammo into different batches.

ivanthehunter

murph226 wrote: »

Normal as in spring operated?

Yes spring operated, sorry about that murph, i should have mentioned that.

Grizzly 45

Term is ; Bolt/slide lock.Used on quite a few suppressed guns,to reduce the mechanical noise of the gun when it is fired.

sfakiaman

An interesting test for those with the Krico would be to shoot groups in bolt and semiauto modes and observe the change in point of impact. This should give an indication of velocity difference which could be used to calculate velocity drop.

Jonty

sfakiaman wrote: »

An interesting test for those with the Krico would be to shoot groups in bolt and semiauto modes and observe the change in point of impact. This should give an indication of velocity difference which could be used to calculate velocity drop.

I did it before and came to the conclusion that under normal hunting ranges, there is no difference in POI. It is concievable that at extreme range, there is a difference in POI, but to determine that the difference is due to the action of the rifle as opposed to other factors