Blowback carbon deposition (rifle case neck)

[Intheshop]

Reading,or should say,re-reading...Tony Boyer's book,pg 149

Ideally,there should be a sine wave of carbon deposit on fired case necks....bout midway down the neck.

22-250 here..

.2255 as cast (Lee 55)

.253 OD neck loaded,with above.

Case comes out @.255,two firings.Not annealed...yet.No turn on case neck.

I'm getting carbon soot about what TB describes.The question is,considering he lives in jacketed world...uhh,we're runnin cast....Do you'all pay any attention to blowback carbon patterns on necks?

.2255 bullets are a moderate sliding,not tight fit into,fired case necks.

 

[Brad]

Never really considered it. If I see carbon clear down the neck and onto the body it is a low pressure load. As pressure rises the black goes a shorter distance down the case.
Not sure it is entirely relevant to us. He is running max pressure loads and looking at it as an indication of a specific pressure rate. We rarely run that kind of pressure with cast.
It would be interesting to see what a really good BR shooter could do with cast. Someone of Tony's ability at the loading and shooting bench would be able to get the most out of a cartridge and bullet.

 

[Ian]

Usually cast loads obturate the chamber from the case mouth back. If there's carbon on the neck either the clearance was too much, the load not warm enough, or the lube bridge failed. If you get sooty cast load necks at high pressure you likely have some other issues as well, such as bullet washout and lube blowout. Thicker necks, possibly larger bullet diameter, and definitely a lube with higher initial viscosity is called for.

 

[Intheshop]

Oh,and BTW....I weighed some of the above bullets.Something that I haven't done in a long arse time....

If I had to put a % figure on,maybe 1 in 10 is "worth" culling.The bullets look good,and weight checking confirms the visual.

 

[Intheshop]

CRS setting in....got some nice 4198,2600 fps ....really good groups cut out of target,staring me in the face on loading bench.

But sitting here stuffing 4895 in?I think it's Fiver's fault.

 

[Intheshop]

OK,that 10 rds went down range 1g more 4895.Not a whole lot different on the blowback pattern.

Recoil pulse is sweet with 4895 so,Fiver is still in the game.

Too dang cold to set up chrono...going 1 more grain.

Gotta say,how well a shooters bench manners are,really do show up on paper.Sounds pretty doh...but don't ignore,or underestimate how important overall balance of your rig/system is to shooting bugholes.

 

[Intheshop]

Another 10,

Very subjective here,but I know this rig.This load went from recoil "push" to,now it's getting interesting.Hazard to guess,2400 fps? range.Didn't weigh bullets,choosing visual.Cpl shots out of the group,but this load is tearing a nice single hole.

Shot 10,at a nice...med fast pace.Super skinny brrl loves cold weather.

Carbon blowback moved noticeably"shorter"....twds case mouth.

Add another 1 g.

 

[9.3X62AL]

Cool thread content. Many thanks.

How well I remember the heavy smoking my now-departed HK-91 gave to its fired brass via the fluted chamber, which was meant to act as an "assist" to extraction by overcoming adhesion-under-pressure of case walls to chamber walls in that roller-locked/recoil-operated 308. It sure threw brass a long dang way, too. Not that it was worth retrieving once located, bent by the port edge as it departed on its 30-35 meter 2 o'clock path of concealment/secondary missile effect. 24 hours of tumbling later, it was de-carboned but still burnt, and prompting those cases into a sizing die was a work-out. The rubber & metal-clip H&K port buffer helped slightly, reducing case bending and departure distance about 25%. After a year of this action, I sold the contraption to a buddy at work for what I had in it and counted the matter as a net gain.

 

[fiver]

I was looking at some 308 brass [yeah I know I been looking at it for 2 week now]
but some of it was used with jacketed and some with cast.
[I know the little mark your talking about]
the just annealed jacketed load isn't quite exhibiting a half way down ring, and the cast loads [with 2400] is showing a darker soot that comes almost all the way down the neck, it stops right at the neck-shoulder junction.
both cases have been minimum sized and necked for their appropriate bullet diameter to give right at .002 neck tension.
I think what this mostly shows is that pressure is opening the case, but gas travels faster and gets around the neck getting trapped between the case and the chamber.
cast will show this differently because it slides into the barrel easier, and is run at a lower pressure.

 

[GaryN]

Cool thread content. Many thanks.

How well I remember the heavy smoking my now-departed HK-91 gave to its fired brass via the fluted chamber, which was meant to act as an "assist" to extraction by overcoming adhesion-under-pressure of case walls to chamber walls in that roller-locked/recoil-operated 308. It sure threw brass a long dang way, too. Not that it was worth retrieving once located, bent by the port edge as it departed on its 30-35 meter 2 o'clock path of concealment/secondary missile effect. 24 hours of tumbling later, it was de-carboned but still burnt, and prompting those cases into a sizing die was a work-out. The rubber & metal-clip H&K port buffer helped slightly, reducing case bending and departure distance about 25%. After a year of this action, I sold the contraption to a buddy at work for what I had in it and counted the matter as a net gain.

I also had an HK but mine was the 93. All the things you described were true of the 93. I only kept it a year or two.

 

[Intheshop]

I believe the original intent for checking amt,and shape is an "assist" in determining case neck clearance.Bushing dies come in many sizes.You can do the math and metrology 'till the cows come home but...

In some areas,a clue,in this example,soot tracks....may trump all the math and subsequent measuring.Like we CB'ers will look past some things,if we know or feel the target is "proving" the load?

We all know bullet fit is king on CB's,neck clearance can fine tune the launch a bit more.I've done some neck turning in the past on .30's,in factory chambers when using bullets .003-.004" over.Not gonna say it's a waste of time because it isn't.But,there's other ways to approach the problem.

Like weighing bullets,neck turning does clear the table for further study.And will continue it when appropriate.This particular rig is NOT a bench gun.

If you've done much reading on BR bench etiquette,you'll run into the notion that something as simple as thumb placement and/or pressure can send tuning out to lunch.Assuming you have a rig capable of showing it on paper.This 22-250 shows it rather clearly.

Buds bags;with interesting loads,2400-2600 fps.If I REALLY put my head down and finesse the shot,they'll rip a hole @100.Back off the manners,one bit,and it shows up as "flyers".....which really aren't.

We've seen it in bow tuning.

Everyone is so dang concerned with load development that they forget about shooting.You,your bag set,stock "cut" and weight,along with...balance,have as much to do with acc as a lot of other concerns.Which seems obvious,but doesn't always get the attention....hard to explain.

One reason Tony recommends shooting/practicing in lousy conditions.You have a built in excuse,IMO.You've done the load development,now work on wind and finessing the rig.....things that aren't as apparent when only shooting on Blue Bird days.

 

[Chris]

Bill, I have a Marlin carbine in .44, best I can get with peeps is 1.5" at 50 yards. However it is a stinker to shoot off the bags, move my thumb just a bit and it will throw an inch or more out. I really agree with you, bag manners are an important consideration. The human factor can be very important.

 

[Intheshop]

I can send you a "Bill"......in fact,it's one of my favorite things.But the given name is Brian,haha.

BUT,prefer BW.

And don't forget bipods'.....we got a bunch of them.That's all we used on varmint rigs for years.They can sometimes be the hot setup with cast.One of my favorite rigs is an early 700VS .308 with cast @1800 fps.On a cheapy Harris bipod it's almost boringly accurate.We use it as a "loaner" of sorts.When guys came over that have never even thought of cast in a rifle...and get that cocked doghead look once discussed.Bring out the VS,tell them to hold ? into the condition....

10 rounds later,their jaws need closing.