Badly pitted bolt face - poor grouping.

[2cants]

HELP Needed.

I'm at my wits end. I've struggled to get the MP10 to consistently group from day 1. Including factory ammo and hand loads.

I've been hand loading for many years (.223, .308, .338) and was expecting 1" groups from reading the MP10 reviews. To be fair I've had one x 0.5" group - the rest are 1-3" groups over 100m.

I've never exceeded the ADI 2208 loading max guidelines, and used once fire ex military brass (PSD - Korean). Max ever load was 5 shots of 46gn AR2208 (Varget) on 168 A-Max's.
Typical load for a competition was 500 rounds of 39gn AR2208 with 165gn Sierra GK.

Now after around 700 rounds - the rifle still isn't grouping (I shoot .2" groups with my TRG and do 1000m+ long range shooting - so I can typically shoot .5" groups all day).

My best group 0.5" was using 2.780" OAL with 39.6gn AR2208 (Varget) with Sierra 165 Gamekings, WinLR primers on PSD case (trimmed/de-burred). This was however a one-off group of .5" and I've not been able to repeat this since.

To make things worse - cleaning my rifle after this weekends failed load development I find my MP10 bolt is pitted badly.
I noticed 4 primers had pin-holes between case and primer cup (with carbon deposit around primer cup). I didn't think it would destroy my bolt face.

I've tried 168gr Amax (which shoot amazing out of my TRG 22), and also tried Berger 175gn, plus Sierra GameKing 165gn. Also tried 150 Sierras. Nothing has been spectacular except for that 1 x .5" group (what a tease).

Since day one I've noticed strong ejector markings on the spent brass (factory loads and hand loads).

Is the pitting affecting accuracy?
Do I need to buy a new bolt or repair my one?
Does S&W supply replacement bolts?

Using once fired brass, Winchester Large Rifle primers (well seated) and never exceeding max powder loads - I don't understand how this could cause over pressure issues.

Any help appreciated.

 

[MistWolf]

If you are pushing the shoulder back too far, it's possible that your reloads are giving you excessive headspace. This will allow the pressure to push the primer out of the pocket slightly, gas to leak around the primer and cut the bolt face. When the case pushes back against the bolt face, this reseats the primer. It is often the case when the load is under-pressured.

It could also be the result of the chamber being too large in diameter at the base. This will allow the base of the case to expand to loosen it's grip on the primer which again lets gas out to cut the bolt face. (Or the base of your cases are too small in diameter.)

I see you're in New Zealand. That may cause complications for you, but it looks like you need to contact the S&W customer service or take your rifle to a New Zealand S&W authorized repair station. What you have is very abnormal

 

[MichiganScott]

Congratulations. I've collected military surplus firearms for 30+ years and that bolt is pitted more than any other I have ever seen. That's not going to be the cause of your inaccuracy, but it's indicative that you have some problems with your loading that will cause inaccuracy.

 

[2cants]

Thanks for the replies Mistwolf and MichiganScott.

I heard some good and interesting news today.
1 - the reseller will talk to S&W to see if they have seen this before and maybe replace bolt under warrant (which I doubt).
2 - My gunsmith says the bolt face can be repaired quiet cheaply by TIG welding and then milling & polishing the bolt face (it might be 1000th of inch less - but still well maintaining headspace). He has done this many times...because...
3 - Seems a bad batch of Winchester LR and LR Magnum primers made their way to NZ and have caused a spate of these issues in last couple of years. Since his other clients moved away from Winchester primers to Federal, Fiocchi or CCI then the problems stop.

So I will get bolt repaired but I'm guessing it still won't be accurate just as MichiganScott suggests.

He did suggest a couple of things to test once bolt repaired...
1 - remove all crimp from the seating process (I use RCBS AR10 dies in a Forster coax);
2 - make oversized projectiles that touch (just off) the lands (don't worry about the magazine fit) - if the accuracy improves then might be too much jump. If so switch to blunt nosed bullets and set to max that magazine will allow;
3 - Use CCI LR primers.

I wonder if anyone else has heard about bad batch of Winchester primers????

If that doesn't work then I'll sell the rifle and get something else.

Thanks again.

 

[copemech]

I cannot resist chiming in on this, although I claim to know nothing.
Used Korean MIL brass? REALLY?

Obvious leakey primers, and it is already screwed, so run more with different primers if you must, BUT?

What does it do with decent factory ammo?

In my observations, the gun is a skinny barrel, and thusly my expectations are set at 1.5 MOA with decent ammo, and if you do better, well?

All said, mine is totally screwed, throws 6-8 inch groups, and is going back to factory this week. I wish it would do 3 MOA, ha!

Sorry,

 

[2cants]

Thanks Copemech

The PSD brass is used by the NZ military and fired in the new LSW C9 gun ammo. I should state that this is South Korean -not North Korean but I hear yah. I only use Lapua for my sniper rifles (TGR22 & 42), all at well below max pressure and also show some (minimal pitting) from Winchester primers.

My shooting partner has a DTA rig (with 3 interchange barrels) and he also has major pitting on his 300WMag bolt face from using Winchester. He is not a happy chappy.

I used the MP10 this year in a civilian sniper competition (as spotter/operator we get to engage steel targets from 50-600m at various elevations & positions, whilst the sniper uses long rifle (typically .338 or 300Wmag) to go out to 1200m. More points for 1st up hits, typically dropping in points for 2nd to 5th round hits. So accuracy is important for a good score.

I chose the MP10 after reading great online reviews saying 0.5" to 1" groups. I also like the 18" barrel for better fps (20" would be better).

I've had the rifle around 10 months, but due to work commitments and being overseas a lot, I hadn't had much time to develop a load. I got several 1.2" groups using 168gr Amax with 39-40grains AR2208 (Varget) at 2.76" OAL. 165Gn sierras GK also went around 1.2"

Factory Winchester 150gr were spayed around like a machine guns beaten zone (these were my first 60 rounds). Nothing looked consistent. I had hoped handloads would have sorted the accuracy - alas no. But nothing came close to 0.5". Many groups around 2".

Copemech, I have read many other forums echoing your comment that 1" to 1.5" groups should be enough. However, from a bench rest with handloads I would expect (hoping for) less than 1".

I've shot in military, civilian and international comps, and am also an army shooting coach - so I'm always trying to improve the accuracy of the rifle, myself or soldier.

Sorry to hear your MP10 is worse. I guess being in the USA should make warranty returns much simpler.

 

[068]

It is possible for a M&P 10 to shoot under 1" groups...mine does. I have tried two brands of "off the shelf" ammo and both have performed well.

This is the first group that I shot. This was using Hornady Match Grade 168 grain BTHP ammo. The four to the right were from zeroing the scope. I believe the center to center measurement was .904". That is 14 rounds, @ 100 yards, under 1", using factory S&W configuration.

This second & third group was shot using Nosler Trophy Grade 165 grain ballistic tips. These were shot after a few upgrades to the rifle. I added a Gisselle SSA-E trigger, MagPul ACS-L stock and a Troy Claymore muzzle brake.

All shooting was done at 100 yards using a redneck engineered shooting table and a Caldwell the Rock rest and rear bag.

 

[2cants]

068, that's the stuff.

Ok, you did some upgrades but not massive (I've done the muzzle brake as the hard plastic stock).

Since I won't use mine for strive rifle comps I can install a trigger upgrade.

Anyone recommend a good trigger?

 

[068]

The Gisselle SSA-E is the only trigger you need.

The trigger and stock I upgraded for myself. I added the muzzle brake for my wife but other than noise redirection it doesn't help much.

 

[copemech]

Thanks Copemech

The PSD brass is used by the NZ military and fired in the new LSW C9 gun ammo. I should state that this is South Korean -not North Korean but I hear yah. I only use Lapua for my sniper rifles (TGR22 & 42), all at well below max pressure and also show some (minimal pitting) from Winchester primers.

My shooting partner has a DTA rig (with 3 interchange barrels) and he also has major pitting on his 300WMag bolt face from using Winchester. He is not a happy chappy.

I used the MP10 this year in a civilian sniper competition (as spotter/operator we get to engage steel targets from 50-600m at various elevations & positions, whilst the sniper uses long rifle (typically .338 or 300Wmag) to go out to 1200m. More points for 1st up hits, typically dropping in points for 2nd to 5th round hits. So accuracy is important for a good score.

I chose the MP10 after reading great online reviews saying 0.5" to 1" groups. I also like the 18" barrel for better fps (20" would be better).

I've had the rifle around 10 months, but due to work commitments and being overseas a lot, I hadn't had much time to develop a load. I got several 1.2" groups using 168gr Amax with 39-40grains AR2208 (Varget) at 2.76" OAL. 165Gn sierras GK also went around 1.2"

Factory Winchester 150gr were spayed around like a machine guns beaten zone (these were my first 60 rounds). Nothing looked consistent. I had hoped handloads would have sorted the accuracy - alas no. But nothing came close to 0.5". Many groups around 2".

Copemech, I have read many other forums echoing your comment that 1" to 1.5" groups should be enough. However, from a bench rest with handloads I would expect (hoping for) less than 1".

I've shot in military, civilian and international comps, and am also an army shooting coach - so I'm always trying to improve the accuracy of the rifle, myself or soldier.

Sorry to hear your MP10 is worse. I guess being in the USA should make warranty returns much simpler.

I hear you man, there is still hope!

I spent the money and freefloated mine with a Midwest Ind. 12" keymod! It still will not hit, but it looks and feels like sex on wheels!

Which scope and Brake are you running? The scope set looks good, as the mount is a bit different to me. What is it?

FYI, on my sucker gun, I have run three different scopes, 3 different brakes, same results, with best group thus far using cheap Tula steel! Insults to injury! And although I have not reloaded in decades I have capability and recently found some fresh supplies which still seem hard to come by such as Sierra Match 168 BTHP and some RL15. I have not wasted the time or effort as yet. Hope it may work one day, cannot Varget! Fed match 168 BTHP was the best I have tried alone with their similar OTM, no glory!

 

[2cants]

I dropped the bolt off today to be rig welded and then milled / polished. Should be back in action in 2 weeks.

Next time CCI primers.

I run Schmidt & Bender PMii 4-16 scope on the MP10.

I think free floating & trigger will get me to 1".

I will keep you posted.

 

[copemech]

Do keep us posted! Mine went back to them this week, hoping for a change for the better without BS! Obviously I did float it, lo-pro gas block installed with a dimple to secure it, and my gas port was enlarged a bit from .068 to .073 to get it to run what I throw into it. We shall see what happens or if they balk or if I have to re-do anything.

No way I would throw a scope like that on a gun that will not hit at 100 with a standard 3x9 Nikon, I can still manage open sights that will do better with my AR15 than the 10 did!

Gee, I love this ****!

 

[LVSteve]

This 308 light bullet problem is not confined to the M&P 10. I have a Howa 1500 heavy barreled bolt gun that does not do well with Winchester 150gr SP and despises M80 ball. The latter ammo produces patterns at 50 yards that are LARGER than my bitsa Century FAL prints at 100 yards.

The same 147 gr M80 shoots like a laser from a converted Chilean Mauser 1912 long rifle, so what is the difference, other than the Mauser wears a 29" barrel.

 

[2cants]

Picked up my repaired MP10 bolt yesterday.

Very impressed with our local Gunworks.
www.gunworks.co.nz
Cost - around USD$50 to repair (mates rates).
Note the polished & bevelled extractor.
Plan to test with CCI primers later this week. I will post results.

Repaired:

Prior With Pitting from Winchester primers:

 

[bergermeister]

Nasty business

Worst cutting I have ever seen. Did the primers drop free from a stretched pocket? I have access to a ballistics calculator and ran a couple things just for kicks. Keep in mind these are CALCULATIONS.

A 168 grain Sierra HPBT loaded to an overall length of 2.80 inches-
PS-80 Military casing (54.1 gr H2-0) Loaded with VARGET 46.0 gr generates 71,990 psi and produces 2700 fps. PRETTY HAIRY

R-P .308 casing (56.5 gr H2-0) Loaded with VARGET 46.0 gr generates 64,153 psi and produces 2637 fps.


A 165 grain BARNES X-BT loaded to an overall length of 2.80 inches- Into the same PS-80 casing with 46.0 gr of VARGET will get you 82, 224 psi and 2783 fps velocity. [I only punched this in to evaluate a 165 grain bullet without having to set up all the parameters of the 165 you used and expected the pressure to drop but it went UP. Would guess that is due to the solid bullet BARNES has in their X-line]

I have found that keeping the pressure below 50K gives good velocity, and increasing above that risks exponential pressure spikes for the minimal velocity increases. JUST MY OPINION

The one example of an MP-10 I had a chance to have the barrel out of had what I would call excessive 'play' between the barrel extension and upper receiver. This can not be conducive to great accuracy. I have read that working a .001 (1-thou) brass shim into the receiver when the barrel extension is seated will help remedy this situation. These are examples where proprietary design bites your butt!

 

[cjwils]

Years ago I got a lot a 1000 primers from a major maker, and I had similar problems. The maker finally confessed that they had produced a large number of primers with metal that was too thin and primers were blowing out with little pin holes in the shoulders. But the damage to my gun was already done.

 

[copemech]

Looks like a great job to fix it! Mine is still a S&W for the accuracy issues.

I still have found no Varget, yet I did find a new source locally that I need to ck with. I did find some RL15 and from what I have read it has good reports in the .308. Any experience to share?

 

[2cants]

Range Report - Progress...but...

Went to the range last Friday and tested the repaired bolt with CCI primers & new Timney AR10 trigger.

Importantly - NO pitted bolt or burst primers.

The new trigger works great, much improved groups.

Shot at 100m, from a bench rest, using 4-14 S&B PMII.

Now to come clean... These where the best of 10 x 5shots groups. Some where WILD groupings (worst group was 3")
The handloaders was busy and we had many discussions.
1 - cases were trimmed and weighed. Extreme spread was from 17fps to 60fps across the 50 rounds (once weighed).
2 - there were no pressure signs (each case inspected)
3 - POI shifted dramatically depending in pressure of the butt into the shoulder (I mean by inches).

BUT - the rifle has potential.

Next step is to free float the barrel.
http://www.brownells.com/rifle-parts/forend-amp-handguard-parts/handguards-amp-rails/308-std-length-free-float-tube-sku231000314-45306-102220.aspx
I hope it will fit.
It's not what I ideally want - but the USA laws mean I cant export any nice tubes more the USD$100, and NZ doesn't have much in the way of DPMS parts.

 

[MistWolf]

The Big Smith uses the same barrel nut threads as the DPMS 308 low height pattern, which is 16 threads per inch.

I'm not sure what makes the DPMS high height different

 

[Bassaro]

Quit while your ahead man. I have been chasing the dragon so to speak with getting my M&P 10 to group well since I got it. At first I thought optics, so I put a leupold vx ii 2-7 on it to replace the prostaff as well as the weaver tac rings I've used on several builds. Still poor grouping so I moved to the trigger and put a Geissele in it, which is a nice trigger but still poor grouping. So finally I moved to a to a Midwest industries free float which looks awesome but solved nothing.

I am a reloader so let's just say I've shot a lot of different loads through it. I began to question my reloads and got some Hornady match ammo...still poor groups!

Once you accept the rifle for what it is your better off. It's a carbine not a long range precision rifle. And much like you I believe the platform has/had potential but it seems like smith put out more lemons with this one then winners, Every time I get on this forum I see failure to feed threads or other problems.

I'm going to give her one last go though. Gonna slap a 4-16 Nikon Monarch on it I have laying around and go heavy this time 180 sierra BTHP.