Bullet set... forward?

[MrRob96]

I'm running into an interesting phenomenon that I'm not finding much info on the web and wondering if others have found the same issue and what they've done.

I'm reloading 7.62x51 for a semiauto rifle (scar 17s) and I'm seeing bullet set forward.

After chambering a round loaded with a 168gr Sierra Matchking, the round comes out of the chamber about .020" longer than it went in. At first this confused me, then I figured the sudden stop once in the chamber is acting like a bullet puller.

I measured a surplus L2A2 round after chambering and noted that bullet grew as well after chambering, tho not as much. It grew about .009". Lighter weight and the bullet has a cannelure where the Matchking doesn't.

Has anyone noted this before and what did you do about it? I can't imagine this helps accuracy.

Read More

 

[Goosebrown]

Why are you removing the cartridge from the chamber? If you chamber it and fire it you'd be ok and never notice.

I think what you are running into is a short lede meaning rifling close to the throat which is probably designed for 147/150 grain military cartridges where the bearing point is shorter.

The shape of each bullet is different as you can tell. With the 168 SMK being 20grains heavier my guess is that it is a little larger diameter where it meets the lands (the bearing point) when chambered so when you unchamber a round the bullet sticks in the barrel a little more.

Is this a problem?

Probably not.

Handloaders do all sorts of things with bullet seating depth and lots of times they jam the bullet into the lands. Others like theirs .002" off the lands and all in between.

Can you fix it? Seat the bullets deeper so they don't jam in the lands.

There is a lot more to the conversation but I think you're ok overall

If you really want to be safe shoot 147/150 GRU military loads

 

[MrRob96]

The L2A2 I tried loading is a 147-8gr bullet. I was comparing the two.

I am trying to load to achieve high accuracy with the scar platform, which it has a reputation for. Hence the SMK bullets.

I'm thinking the set forward may actually impact that accuracy because the creep forward will be inconsistent I imagine.

 

[oli700]

Interesting way you found to get around mag length match ammo , if your lucky they are stopping .010 from the lands lol . Makes sense though and I bet reloads that aren't on point with neck tension would account for a little inconsistency in the group due to in inconsistent length from the lands .it would be interesting to compare groups against single hand loaded shots see if there is something to it .

 

[DizzyJ]

My guess is neck tension if you chamber, extract, and the OAL is longer. More neck tension should hold it tighter, but there's a limit.

 

[66PonyCar]

I'm thinking the set forward may actually impact that accuracy because the creep forward will be inconsistent I imagine.

I think the forward creep only occurs when you extract the chambered round. If the bullet is engaging the lands then when you extract it is acting like a bullet puller. So what kind of accuracy are you getting from the reloads?

 

[ConcernedCitizen]

If you chamber it and fire it you'd be ok and never notice.

I'm going to strongly disagree with this comment.

If every single bullet jumped forward the same amount while working up your load you might be okay, but there is no way I would trust it.

There are two distinct ways to spike the pressure of an otherwise safe cartridge; bullet setback (reducing case volume), and jamming a bullet into the lands (increased pressure needed to overcome friction and swaging from a dead stop). Neither condition is safe, unless deliberately worked up from scratch.

OP, you are correct that inertia is what's likely driving the bullets forward. It's also possible the bullets are being physically pulled if they are getting wedged into the lands, as someone else previously mentioned. The easiest way to check, if you're not already seeing marks in the jacket, is to blacken a bullet with a sharpie or with soot. You'll clearly see if it's contacting the land or not when you remove it. Contacting the lands is bad.

The two easiest solutions are increasing neck tension or crimping.

Increasing neck tension is fairly easy. Remove the expander ball from the sizing die, chuck it up in a drill, and use emory cloth to carefully reduce the diameter.

For crimping, I would avoid a roll crimp due to lack of a cannelure, and would try using a Lee Factory Crimp Die instead. They use a collet to taper crimp, rather than a roll crimp.

I personally would not continue until you can address this issue, as having bullets wedged into the lands is unsafe. It causes excess pressure, and when removing live cartridges it can result in removing the case but not the bullet, if it's wedged in there tight enough. This results in an action full of powder and a bullet that needs to be ramrodded out of the barrel.

Please keep us posted on the progress!

 

[ConcernedCitizen]

I'm thinking the set forward may actually impact that accuracy because the creep forward will be inconsistent I imagine.

Your chamber pressure will likely be all over the place, affecting velocity. I'd be curious to see how much it affected standard deviation. Have you been chronographing these by chance?

 

[Goosebrown]

I think the forward creep only occurs when you extract the chambered round. If the bullet is engaging the lands then when you extract it is acting like a bullet puller. So what kind of accuracy are you getting from the reloads?

Exactly. Not creep but pull.

Don't extract and all the rounds will fit the same in the lands. It may well increase accuracy as they'll all be the same.

A 168smk might or might not be more accurate in your rifle. You might want to test 150 match bullets. There is a 150smk. The scenar 155s are legendary and they'll be less likely to hit the lands. But just seating your bullet deeper will fix your problem.

Also the OAL is related to magazine length not fit in the chamber. For that you want to measure the mentioned bearing point. Get a Hornady bullet comparitor to measure that in YOUR unique chamber.

 

[Goosebrown]

I'm going to strongly disagree with this comment.

If every single bullet jumped forward the same amount while working up your load you might be okay, but there is no way I would trust it.

There are two distinct ways to spike the pressure of an otherwise safe cartridge; bullet setback (reducing case volume), and jamming a bullet into the lands (increased pressure needed to overcome friction and swaging from a dead stop). Neither condition is safe, unless deliberately worked up from scratch.

OP, you are correct that inertia is what's likely driving the bullets forward. It's also possible the bullets are being physically pulled if they are getting wedged into the lands, as someone else previously mentioned. The easiest way to check, if you're not already seeing marks in the jacket, is to blacken a bullet with a sharpie or with soot. You'll clearly see if it's contacting the land or not when you remove it. Contacting the lands is bad.

The two easiest solutions are increasing neck tension or crimping.

Increasing neck tension is fairly easy. Remove the expander ball from the sizing die, chuck it up in a drill, and use emory cloth to carefully reduce the diameter.

For crimping, I would avoid a roll crimp due to lack of a cannelure, and would try using a Lee Factory Crimp Die instead. They use a collet to taper crimp, rather than a roll crimp.

I personally would not continue until you can address this issue, as having bullets wedged into the lands is unsafe. It causes excess pressure, and when removing live cartridges it can result in removing the case but not the bullet, if it's wedged in there tight enough. This results in an action full of powder and a bullet that needs to be ramrodded out of the barrel.

Please keep us posted on the progress!

I don't think this is drift forward, the bullet is seated too far out, getting caught in the lands then getting pulled out as it's being ejected.

If they're all jammed then the pressure is more consistent not less.

One way to tell is check the extracted bullet. If you see marks from the lands on it, the bullet is hitting the lands and if you can't then it is probably drift forward.

Beginning reloaded problem with heavier bullet than intended

 

[bsa1917hunter]

I'm running into an interesting phenomenon that I'm not finding much info on the web and wondering if others have found the same issue and what they've done.

I'm reloading 7.62x51 for a semiauto rifle (scar 17s) and I'm seeing bullet set forward.

After chambering a round loaded with a 168gr Sierra Matchking, the round comes out of the chamber about .020" longer than it went in. At first this confused me, then I figured the sudden stop once in the chamber is acting like a bullet puller.

I measured a surplus L2A2 round after chambering and noted that bullet grew as well after chambering, tho not as much. It grew about .009". Lighter weight and the bullet has a cannelure where the Matchking doesn't.

Has anyone noted this before and what did you do about it? I can't imagine this helps accuracy.

I often wonder why guys don't know how far off the lands their hand loads are. That's the first thing I do, before I ever even load a certain bullet. Check your OAL and be sure the bullet is off the lands. I generally back the bullet off by at least .020". If you think the bullet is being pulled from the case after chambering and then extracting the chambered round from the rifle, you are into the lands way too much. The bullet should never be that close or jammed into the lands IMHO. Stay off the lands by a set amount and then try to add a little more crimp to your loads and I bet your problem will disappear.

 

[ConcernedCitizen]

I don't think this is drift forward, the bullet is seated too far out, getting caught in the lands then getting pulled out as it's being ejected.

If they're all jammed then the pressure is more consistent not less.

One way to tell is check the extracted bullet. If you see marks from the lands on it, the bullet is hitting the lands and if you can't then it is probably drift forward.

Beginning reloaded problem with heavier bullet than intended

Land marks on the bullet won't determine if they're from an overlength cartridge or from inertia, as both could potentially drive the bullet into the lands.

Allow me to settle this debate rather simply.

How can OP have a C.O.A.L. long enough to hit the lands, but short enough to fit in the detachable magazine?

A quick search tells me that 2.800" seems to be a common OAL for .308/7.62, with most loads for the SCAR 17 seating to 2.820". One comment said that 2.860" looks like it would hit the front of the magazine, which is good to note.

Another person was kind enough to measure the freebore when loaded to 2.800" OAL. Here are his findings:

Privi 145gr. FMJ projectile: .111" (+-)
SMK 168gr. HPBTM projectile: .085" (+-)
SMK 175gr. HPBTM projectile: .090" (+-)

All measure 2.8" COAL.

(Source: Maximum O.A.L. for .308/7.62 Cartridge in Scar 17s )

So, if we know that the 168gr SMK has approximately 0.085" of freebore when loaded to 2.800" OAL, and we know that 2.860" appears to be the maximum physical length that will fit in the magazine, so that leaves a freebore distance of approximately 0.025" to the lands even if the cartridge is loaded to the maximum physical OAL that could possibly fit in the magazine.

There is no physical way the bullet can be touching the lands and still fit in the magazine.

OP, the bullet is being driven forward by inertia, exactly as you surmised. It may have been driven far enough to contact the lands, which could contribute to the bullet being pulled out further, but I don't think that's the case. Based on your initial measurement of an additional 0.020", I'm presuming that the bullet is stopping short of the lands, but it would still be worth checking with a sharpie or soot.

Having a bullet jump forward like this will slightly lower chamber pressure by increasing case volume, but if the bullet contacts the lands upon chambering, it will increase the chamber pressure. Hornady says this is okay, as long as you reduce the powder charge by a few grains to compensate, but it can be dangerous with a full-powered load.

I still suggest either polishing and reducing the diameter of the ball expander, or using a Lee Factory Crimp Die. If you decide to go with the ball expander, you might consider getting a replacement and modifying that, so you still have your original if needed.

Good luck!

 

[oli700]

this is a scar not a bolt gun ......some of you have sound theories for a bolt gun .

 

[Greenbug]

Why are we trying to get bolt gun accuracy out of a SCAR?

 

[ConcernedCitizen]

Why are we trying to get bolt gun accuracy out of a SCAR?

Why not?

Semi-auto rifles have come a long way, and can be capable of some pretty amazing accuracy. You might as well try to get the most accuracy out of it, rather than just presuming it's a 2 MOA gun.

 

[MrRob96]

Why are we trying to get bolt gun accuracy out of a SCAR?

Why not?

Semi-auto rifles have come a long way, and can be capable of some pretty amazing accuracy. You might as well try to get the most accuracy out of it, rather than just presuming it's a 2 MOA gun.

This. If you check out the accuracy potential of the scar 17, you'll see they're regularly capable of sub MOA accuracy with handloads.

So someone is handloading these things with good accuracy. And I figured this topic applied to all SA rifles so I asked here.

I will do some experiments this morning to see if the increased OAL of the cartridge is due to the lands pulling the bullet or inertia of the bullet upon lock up of the bolt.

 

[ConcernedCitizen]

This. If you check out the accuracy potential of the scar 17, you'll see they're regularly capable of sub MOA accuracy with handloads.

So someone is handloading these things with good accuracy. And I figured this topic applied to all SA rifles so I asked here.

I will do some experiments this morning to see if the increased OAL of the cartridge is due to the lands pulling the bullet or inertia of the bullet upon lock up of the bolt.

The easiest way to check would be to measure a few cartridges beforehand, load a few like normal, and gently load a few by riding the charging handle. That should tell you immediately if it's inertia or not.

 

[eightyeight mag]

Make dummie rounds, load round into chamber then let bolt slam down on it. Does that change seating depth?
If it does then I would be concerned with it jamming into lands.
The scar bolt carrier is very heavy and will definitely cause issues with bullet seating if neck tension is not right.
This is why the scar kills cheap optics. Bolt carrier is no joke.

 

[MrRob96]

Ok, update time.

I ran the following test with 3 different milsurp loads, and once with my dummy rounds, and once with a factory match load: slow close of bolt with hand, and letting the bolt slam closed with bolt release button. All rounds were sitting in the magazine at the time of bolt close.

1. Malaysian L2A2
Factory OAL: 2.7865"
OAL after slow bolt: 2.7865" +0.0"
OAL after bolt slam: 2.788" +0.0015" Light land mark on bullet

2. Korean M80
Factory OAL: 2.789"
OAL after slow bolt: 2.789" +0.0"
OAL after bolt slam: 2.792" +0.003" Light land mark on bullet

3. SM Surplus (German M80 w/ NATO cross)
Factory OAL: 2.784"
OAL after slow bolt: 2.784" +0.0"
OAL after bolt slam: 2.788" +0.004" No land mark on bullet

4. Dummy cartridge (168gr Sierra Matchking HPBT)
Factory OAL: 2.795"
OAL after slow bolt: 2.798" +0.003"
OAL after bolt slam: 2.813" +0.015" Light land mark on bullet

5. Federal Gold Medal Match (175gr Sierra Matchking HPBT)
Factory OAL: 2.8005"
OAL after slow bolt: 2.801" +0.0005"
OAL after bolt slam: 2.808" +0.007" Light land mark on bullet

In the above data, light land mark means I can see a faint horizontal line on the bullet jacket, and my thumbnail can feel the line as well. I do not think this is holding the bullet enough to pull it on extraction. Also the relatively unchanged OAL of the bullet upon slow bolt closing may indicate the same as well.

It seems like the bullets are being pulled by inertia.

I probably have less case neck tension due to lighter crimp than the military rounds. The SMK also has no cannelure as I mentioned in my first post. Comparing my dummy load to the Fed GMM round, it seems I have more bullet creep compared to a factory load, so my neck tension may be too light. Tho the bullets dont slide into the case neck easily, so I'm not sure how to solve this. That is when I'm seating the bullet, I can see the base of the bullet is bulging the neck when I don't seat fully, indicating the neck is being stretched around the bullet's bearing surface.

 

[Member 5605]

Yes.

The inertia of the bullet being chambered and stopping is making the bullet jump forward.

Crimp and this won't happen