Help with Groups from M305

Pr589

Pr589

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Hey folks. Just came back from the range and was having an issue that I have read others have experienced.

So the rifle is shooting well. 1 MOA groups if I exclude the first round which fairly consistently goes 1 to 2 MOA high. Can anyone venture a theory why this is happening?

My only thought right now is that it is related to barrel temperature i.e. changing mags after a 5 round string lets the barrel cool down enough to introduce the high POI on the first round. I'm not convinced by this theory since it seems like a lot of deviation from a fairly short cool down.
 
Yah I know. Read and participated in some threads but don't recall anything conclusive coming out of them and can't seem to find them using Search. Not even sure if everyone experienced this issue. Seemed only a few saw this. I'm just starting to get groups tight enough that I can really observe this pattern.

Think of this as a second try ;)
 
there's so many variables it's impossible to answer the question on the internet.
some guys have nice groups up till the 5th round.... it's accepted by many that this is caused by change in magazine spring pressure on the 5th round. i tested the theory in a rifle that had the issue and compared the findings between that mag and an aia mag with 10 rounds. groups were better in that rifle with 5 rounds shot from a 10 round aia mag..... the 5th round flier from the 5/20 was no longer a flier from the aia mag. spring pressure seemed to be the issue.

so it's pretty hard to say what would cause a first round "flier" in your five round group. you will probably have to play around and try a few things. if it does this consistantly, just compensate with your sights on that first shot :D
 
Thanks guys.

Yo - That link was a good find. Searched the M14 forum but couldn't find what I wanted. Google is better than the search engine on the site.

After reading the M14 forum post and 45ACP's I'm now wondering about the effect of the magazine. I tried holding low for the first round on one of my strings but wouldn't you know, the rifle shot exactly where I was aiming on that string :(

At least I know that this isn't necessarily inherent in the platform, there is likely something happening with my rest, my mags or me. I think I'm going to try an AIA 10 round mag to see if a longer string, or change in mag has an effect.

See the groups below just for interest.

2013-01-13142419_zpsf17a8c4e.jpg
 
Could be how the rifle feeds from racking the charging handle, vs how it cycles when it fires. Try chambering a round, then putting in a loaded 5 round mag. Shoot the first round off target, then try a 5 round group. Just an idea.
 
Good idea Hey. I've also removed the AFG that I had on the rifle since I think that this might have been in contact with the rest on the first round until recoil pushed the rifle back and out of contact. I'm going to try a 10 round Mag with 5 into one target and 5 into another. Should isolate the mag and feed effect.

Read an interesting post on the M14 Forum from a guy called EBR Builder who used to assemble EBRs for the US military. He says that they used to discard the first and last rounds when certifying the rifles for military issue. Seems that they never identified the causes.
 
You could also put that target up with 5 rounds in your magazine and shoot #1 @ top left target, #2 @ top right target, #3 @ middle target #4 @ bottom left, and #5 @ bottom right. Then load your magazine 4 more times and repeat the pattern. I'd like to see that target.
 
So the idea being that we're testing first/last shot accuracy or shot-to-shot POI relative to POA?

I may do this next time at the range but I think my lack of consistency from mag-to-mag would overwhelm any patterns generated by order-of-firing. Might be interesting though...hmmm.
 
Well, the idea is that as long as your barrel doesn't heat and act up and as long as you don't fatigue, you should have nice MOAish groups and your top left target group would be about 2 MOA higher than your other groups. Just use the one magazine, this is to confirm that the first shot is always high, this is not about speed. If the target comes out as I describe then there is something going on with your rifle that probably has to do with the way the action is cycled. It may be more consistent when it cycles itself than when you drop the bolt on the first round.

Another thing you can try is to turn your gas off and see how a 5 shot group turns out when you cycle the bolt manually each time.
 
SoBored said:
Well, the idea is that as long as your barrel doesn't heat and act up and as long as you don't fatigue, you should have nice MOAish groups and your top left target group would be about 2 MOA higher than your other groups. Just use the one magazine, this is to confirm that the first shot is always high, this is not about speed. If the target comes out as I describe then there is something going on with your rifle that probably has to do with the way the action is cycled. It may be more consistent when it cycles itself than when you drop the bolt on the first round.

Another thing you can try is to turn your gas off and see how a 5 shot group turns out when you cycle the bolt manually each time.
Click to expand...

This is good advice right there ^^^
Flyers are pretty common with this action. So many parts moving and the mag is jigglin. Narrowing it down by trying Sobored's target idea is great. You should see if that flyer is consistantly coming from a particular shot. Ie first shot high only or whether the flyer becomes a random round from the mag stack.
In the end though, it may not be a condition that can be "fixed" due to the variables at play in this particular action.

Another question?
How tight is your gas lock as gas lock tension can also affect group consistancy as the barrel and gas system heat up.
 
I'll try this ^ on Saturday when/if I get to the range. Might be wet and windy.

45ACP, the gas lock is tight and is timed as recommended on these boards (i.e. 5:30).

Thanks all for the input.
 
Cold bore shots on a semi-auto rifle almost always has the first poi different then the rest of the group. The key is the constitacy. If it constitanatly prints 1.5"'s away from the rest of the group and your log book records prove this then you know to make that adjustment for the first shot then resume the rest of shooting as per normal.
 
So after a long time away from the range, interupted by life, skiing/snowboarding, etc. I finally got back to do some shooting and debugging my rifle. This was an interesting shoot with a few learnings. The first thing I tried was to follow up on SoBored's suggestion in post #9 of this thread. It sounded like an interesting test.

The M305 I'm using is shimmed, M14.ca op. rod spring guide, Blackfeather stock, staked gas cylinder assembly (thanks to Hungry), Vortex flash hider and has a 3x9 Zeiss scope mounted. Rifle headspaces at a big 17 thou above spec. The temperature was hovering about 0 degrees. The rifle started clean and was warmed-up and fouled with 40 rounds of MFS (just having some fun. Was surprised at how big the first groups were - 6" for the first mag, then started to shrink until the usual 2" to 3" grouping by the 4th mag). Targets were changed and a further 5 rounds of MFS was shot to warm the barrel. I then loaded a mag, with 5 rounds of Hornady 155 gr. A-Max Match and fired one round into 5 separate targets in sequence (i.e. 1 round at target 1, round 2 at target 2, etc.). I reloaded and did this again with 3 additional mags. See the results below.

Sequence_zps2b7b3d09.jpg
[/IMG]

Some observations - POI was clearly low and to the left while it was zeroed for MFS ammo (I can understand a velocity difference but the skew to the left is curious). The POI for the first round from the mag is higher and has greater stringing than for the other rounds. While not marked in the pic, I observed that the pattern in 1st round impacts moved from 1.5" to 2" high (relative to POI from rounds 2 to 5), downward so that by the last string the POI was about where the other rounds were hitting. Nearing the end of the 4 strings I had to slow down my pace because the heat shimmer from the barrel was making it hard to be certain of the POA.

At this point I think that the first round flyers is likely due to just barrel temperature effects (at least when using this mag). If this is the case, I'm surprised that starting with a warm-to-the-touch barrel, 5 rounds aren't enough to bring the barrel to "operating temperature" and also that the brief time needed to reload a mag, would allow enough cool-down time to affect POI on the first round.

Anyway, the first-round effect was looking kinda consistent so after pondering for a bit, I shifted to my second clean target (no need for a walk up to the target stand) and tried some groups holding low for the first round (and tweaked my scope to bring POI in alignment with POA). The results of the 4 strings are in the next pic, numbered in sequence. I started holding 2" low for the first round and them moved to 1" low for the rest. There was more spread in the first group (perhaps due to temperature effects) than the next 3 groups. By the end the barrel was again quite hot with heat shimmer affecting the sight picture but I was quite happy with how the rifle was performing. Too bad I was out of the Hornady ammo although I picked more up on the way home (need the brass anyway).

Next step is some bolt swapping to try to find a tighter fitting bolt and reloading to reduce cost and experiment with COAL, powder loads and bullet weight.


Groups_zps7c6b6403.jpg
[/IMG]

So what do you guys think? In your experience does barrel temperature have this big an effect and would a medium contour barrel make temperature effects more easily controlled? (You know I'm just looking for an excuse to go for a Krieger medium profile barrel)
 
Pr589 said:
So after a long time away from the range, interupted by life, skiing/snowboarding, etc. I finally got back to do some shooting and debugging my rifle. This was an interesting shoot with a few learnings. The first thing I tried was to follow up on SoBored's suggestion in post #9 of this thread. It sounded like an interesting test.

The M305 I'm using is shimmed, M14.ca op. rod spring guide, Blackfeather stock, staked gas cylinder assembly (thanks to Hungry), Vortex flash hider and has a 3x9 Zeiss scope mounted. Rifle headspaces at a big 17 thou above spec. The temperature was hovering about 0 degrees. The rifle started clean and was warmed-up and fouled with 40 rounds of MFS (just having some fun. Was surprised at how big the first groups were - 6" for the first mag, then started to shrink until the usual 2" to 3" grouping by the 4th mag). Targets were changed and a further 5 rounds of MFS was shot to warm the barrel. I then loaded a mag, with 5 rounds of Hornady 155 gr. A-Max Match and fired one round into 5 separate targets in sequence (i.e. 1 round at target 1, round 2 at target 2, etc.). I reloaded and did this again with 3 additional mags. See the results below.

Sequence_zps2b7b3d09.jpg
[/IMG]

Some observations - POI was clearly low and to the left while it was zeroed for MFS ammo (I can understand a velocity difference but the skew to the left is curious). The POI for the first round from the mag is higher and has greater stringing than for the other rounds. While not marked in the pic, I observed that the pattern in 1st round impacts moved from 1.5" to 2" high (relative to POI from rounds 2 to 5), downward so that by the last string the POI was about where the other rounds were hitting. Nearing the end of the 4 strings I had to slow down my pace because the heat shimmer from the barrel was making it hard to be certain of the POA.

At this point I think that the first round flyers is likely due to just barrel temperature effects (at least when using this mag). If this is the case, I'm surprised that starting with a warm-to-the-touch barrel, 5 rounds aren't enough to bring the barrel to "operating temperature" and also that the brief time needed to reload a mag, would allow enough cool-down time to affect POI on the first round.

Anyway, the first-round effect was looking kinda consistent so after pondering for a bit, I shifted to my second clean target (no need for a walk up to the target stand) and tried some groups holding low for the first round (and tweaked my scope to bring POI in alignment with POA). The results of the 4 strings are in the next pic, numbered in sequence. I started holding 2" low for the first round and them moved to 1" low for the rest. There was more spread in the first group (perhaps due to temperature effects) than the next 3 groups. By the end the barrel was again quite hot with heat shimmer affecting the sight picture but I was quite happy with how the rifle was performing. Too bad I was out of the Hornady ammo although I picked more up on the way home (need the brass anyway).

Next step is some bolt swapping to try to find a tighter fitting bolt and reloading to reduce cost and experiment with COAL, powder loads and bullet weight.


Groups_zps7c6b6403.jpg
[/IMG]

So what do you guys think? In your experience does barrel temperature have this big an effect and would a medium contour barrel make temperature effects more easily controlled? (You know I'm just looking for an excuse to go for a Krieger medium profile barrel)
Click to expand...
Yep thicker barrel between the gas system and the reciever will resualt in tighter groups. This is becuase the less barrel whip on a semi-auto piston platform the higher chance of repeatability threw lowering the harmonic inblance that the piston causes when slaming the op-rod wile the barrel is whipping.
Although the road you are about to go down will be costly and a bolt action will still achieve better groups but minus the speed that you can shoot this semi.
 
Was the groups a over achieved with hand loads or factory ammo??

Perhaps before you launch in to a custom barrel, if this was using factory ammo. Give reloading a try, as now all your once fired bras will be fire formed and you can try a lot of different combinations to see what your rifle likes. I Found some good success with 155 Amax bullets.
 
Buzzballer said:
Was the groups a over achieved with hand loads or factory ammo??

Perhaps before you launch in to a custom barrel, if this was using factory ammo. Give reloading a try, as now all your once fired bras will be fire formed and you can try a lot of different combinations to see what your rifle likes. I Found some good success with 155 Amax bullets.
Click to expand...

It's my understanding that the action works so fast on a m14 that it extracts the casing before it can fully expand and contract therefore no brass can be fire formed in a m14 only fire deformed ... This is why the brass can only be loaded a few times before it starts to show case head separation also why you should use 7.62x51 type casings they are thicker walled and can stand the abuse
 
Bakersfield7.62 said:
It's my understanding that the action works so fast on a m14 that it extracts the casing before it can fully expand and contract therefore no brass can be fire formed in a m14 only fire deformed ... This is why the brass can only be loaded a few times before it starts to show case head separation also why you should use 7.62x51 type casings they are thicker walled and can stand the abuse
Click to expand...

This might be a place for some one with more experience then I with reloading to chime in. The expansion of a case from firing happens at the time of controlled explosion of its propellant. The combustion of the propellent forces the brass case to expand and creates a seal around the fireing chamber. If the case did not expand, you would have a leaking of gas coming back past the case and in to the bolt and possibly the operators face. The exhaust gas from that explosion then pushes the bullet down the barrel. Then a small percent of gas is utilized to cycle the gas operating system, extracting the round and chambering a new round. The expansion of the case happens long before the round is extracted.

We are able to prove this when we use a case micrometer to measure the expansion of the case to show us the chamber size. From my understanding the main problem with reloading brass from a semi auto is that it's chamber is typically more lose then a single shot rifle. Requiring more brass to be pushed around every time you fire and reload the case. Eventually the brass around the neck becomes thin and can crack. This is especially true if you just use a neck resizing die instead of using a collet die. Some semi auto rifles (like the mini 14) cause the case to be bent some times on extraction. But most m14 (read M305) are fairly gentle on the ejected brass, even making a nice pile of it.

7.62x51 NATO rounds are thicker walled cases, and require less powder to get to the same pressure as commercial .308 cases. But typically reloading military rounds can be a lot of headaches, the brass is usually not any where as good as match commercial brass, quality of material and consistency of dimensions from case to case. Not to mention that they usually have crimped primer pockets making it very hard on the first primer extraction. Aswell they were designed to work in full automatic rifles so they must be made to a very lose tolerance to be able to fit in any dirty chamber.

Hand reloading is one of the biggest ways you can unlock the accuracy of your rifle.
 
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