80 grain SMK's breaking up.

[ovenpaa]

My wife has been very successfully shooting with the .224 SMK 80grain moly coated bullets for the last three years. She shoots a wildcat 6,5x47 Lapua case necked down to .224 with a no turn neck. The Action is a Savage Match bedded in a BVSS stock with a 29" TruFlight 1:8 barrel and she typically shoots at around 3250 fps with H4831SC with very good results. She and the rifle have shot .019" and better at 100 yards however she uses it as a 600-1200 yard rifle. Yesterday she was shooting at 800/1200 yards with the rifle.

The day was hot for us at around 30C/85F and for the first time ever she had some of the bullets disintegrate, this is typically seen as a small puff of dust/smoke at around 150-200 yards out from the firing point followed by no hit on the target, we know she was shooting OK as she she was grouping nicely on the 1200 yard target so certainly something was going wrong. Given the ambient temperature her MV would have been up a bit but she was not seeing any significant pressure signs so my question is has anyone heard of this happening with the .224 SMK80 before and does anyone have any view on what the maximum MV and rotation the bullet will run to is? I ask because we are talking about building another wildcat for her based on the same bullet.

She has shot on hot days before so it was really odd to see this happen and is a first for any of her rifles. We have absolutely no plans to move away from the SMK for the bullet as it has worked so well for countless hundreds of rounds.

Does anyone have any views on this?

 

[JASmith]

Every bullet has a different max rotation rate, depending on details of design and manufacture.

One might think that you and your wife have experimentally determined the maximum rotational rate for that bullet. It is simply the number of turns per foot times the muzzle velocity. Multiply by 60 if you want it in RPM.

How much do you lose in real wind-bucking at 1200 if you find an accuracy node 50-100 ft/sec below the the current load?

Is this enough to be a real problem in ability to get decent scores?

 

[xswanted]

I'm shooting 80SMK's out of my 22-250AI at 3365fps with no issues with anything as far as breaking up is concerned. 26" barrel.

Could it possibly be a bad batch of bullets?

 

[Louisiana Shooter]

How many rounds down that barrel?

It could be fire cracking that is starting to effect the bullets.

 

[DOGCAPPER]

Rounds in direct sun with hot loads will run up past the red line and puff blue smoke..Ask me how I know...LT

 

[ovenpaa]

The rifle has 837 rounds through it with 42.1 grains of H4831SC which gives around 3300fps so not an unduly hot load, same batch of bullets and she buys her powder 5 tubs at a time and then mixes it all together in a bowl (In the kitchen with a wooden spoon!) She did keep the bullets shaded however she said the rifle was hot. Maybe this is a simple of matter of hot rounds over pressuring however I will put a bore scope in the rifle to see what the throat looks like.

I have mailed Sierra to see if they have any views on maximum MV and RPM for these bullets however I must say they are always very good. As far as dropping the load the load development was very short, after sighting in it shot a .194 and .197 so she stopped at that point. Load development was over and done in ten shots.

 

[lpreddick]

i agree that your barrel may be the problem. your cartridge is a barrel burner, esp if shot in competition. a borescope will settle the issue. firecracking and missing pieces of barrel will tear into a bullet and with an 8 twist at your velocity could probably damage the bullet. borescope for sure. let us know.

 

[KevinThomas]

Regardless of the round count (though this may well be a part of the problem), you're pushing this bullet far, far faster than it was designed to go. The SMK 80 was intended specifically for the 5.56mm round, and for being single loaded in ARs for the 600 stage of NM competition. Anything beyond that that it does or does not work in, is purely coincidence or a matter of luck. I suspect you're simply pushing up against the boundries of what the bullet will stand.

 

[ovenpaa]

A belated reply, I have been working every hour of light available and a few dark ones recently..

Sierra tell me

We’ve heard of this happening on very rare occasions.It’s always been something traced back to the rifle or barrel.In other words we’ve seen a burr in the barrel or more times than not a throat has gotten rough and heat checked which damage the jacket.On another note we’ve seen a build up of carbon just ahead of the chamber.If I was guessing,my first guess would be that the throat is the problem.What I’d do is scrubb the throat 7-8 times with JB and see if that doesn’t cure it.

Today we bore scoped the rifle and it looks good, slight throat erroison but nothing to be unduly concerned about. I have scrubbed the bore with VFG paste and cleaned, scrubbed, cleaned and checked again, it all looks fine so next move is shoot it some more. At least summer is over so no more worries about temperatures here.

Thanks all for your comments so far.

 

[effendude]

My wife shoots a .22-243 aka "22 Lisa" for F-class. 1-8" Hart barrel, 30" long. She runs about ~3400 fps with 80-82 grain moly'd bullets. She and I both shot OTC with space guns in the past so I have a huge inventory of moly'd 80 SMKs. She had a few 80s never make it to the target in the past.

I found 500 82 grain Bergers, moly'd, on a clearance rack and she shot all of them this year without a single blow-up. Not necessarily a scientific experiment, but I agree with Kevin's theory regarding the designed purpose of the 80 grain SMK. With my supply of moly'd Berger bullets gone, I am going to rework a load with naked bullets, both 80.5 fullbore and 82 if I can find them.

Scott