240xtp mag testing

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Re: 240xtp mag testing

Postby Hoot » Tue Apr 20, 2021 1:49 pm

Increasing the charge seems like the easiest path to a solution, however you might want to stay with the lesser charges and go with a hotter primer than standard Federal SRP's. Also, a chronometer is essential to successful load development. Group performance is important as a qualitative measurement but a chrony is important for quantitative measurement. It will shed light on the impact that a hotter primer provides. If you can't get your hands on some hotter primers, then increasing the charge would be the next best step IMHO.

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Re: 240xtp mag testing

Postby s4s4u » Tue Apr 20, 2021 6:50 pm

its a fair guess with that light of a buffer that your carrier speed is actually outrunning the bolt catch lift from the mag spring

Not trying to argue, but how does that reconcile with the fact that the gun locks back with the more powerful factory loads?
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Re: 240xtp mag testing

Postby plant_one » Wed Apr 21, 2021 10:36 am

s4s4u wrote:
its a fair guess with that light of a buffer that your carrier speed is actually outrunning the bolt catch lift from the mag spring

Not trying to argue, but how does that reconcile with the fact that the gun locks back with the more powerful factory loads?

ok so.. i'm no engineering expert, so im going to be very generalized based on how i understand the system works. it was something i had to try to wrap my head around when trying to figure out subsonic cycling issues when i first started messing with 300 blackout (right after sandy hook when EVERYTHING was unobtanium like it is now).

so... generally speaking, looking at the powder difference alone

different powder = different peak pressure curve, different amount of gas produced, different port pressure when firing = different carrier speeds while cycling

when we're discussing milliseconds - or even a fraction of a millisecond - in difference in timing of a gas impingement ar's system operation - it doesnt take much for A to work and B to fail.

from the site here (this is a generalized graph)


now when you introduce powder variables into the mix, your pressure curve can look like this (yes i know its not 450 bm.. but i'm going by what i can find)

the important note is the time function of when those pressure peaks hit with the different powders. its part of the reason we use the group of powders we do in our ar's but bolt/single shot guys have a LOT more options on powder - we need to have a pressure curve that gives us a substantial enough volume & pressure of gas left in the barrel once the bullet passes the gas port, but not TOO much either.

even when looking just at factory ammo, pressure curves can vary wildly. Lucky gunner did a test a few years ago on brass vs steel case 223 torture test . part of what they tested was the various pressure curves in the ammo they were firing as well as looking at the port pressure. part of this was trying to figure out why their TULA ammo tested so poorly in the showing. yes they did specify that gas port size in the test gun the tula was fired from was part of the problem, however they also came up with a VERY important consideration as far as powder burn rate goes and the pressure curve that goes along with it.

chamber pressure graphs from that test - again dont so much look at the peak pressure differences, but focus on the timing of when those peaks hit.

federal 223 55gr fmj

tula 223 55gr fmj

their explanation

https://www.luckygunner.com/labs/brass- ... a_analysis
Powder Burn Rate & Gas Port Pressure
What’s really important in this case, however, is not the maximum chamber pressure number, but powder burn rate and thus gas port pressure. Whether measured in clean, fouled, new, or worn out barrels, Tula exhibited gas port pressures that were 10-20% lower than all other ammunition types.

Basically, the powder burns too fast, and by the time the bullet has reached the barrel, the pressure drops. The rise time of Tula, defined as the time in microseconds for pressure to rise from 25% to 75% of maximum chamber pressure, is 175ms. In comparison, Federal AE223, depending on temperature, has a rise time of 260-300ms.

this is a very key thing to take into consideration when trying to figure out cycling issues, especaially when taking variables like powder choice, primer selection, etc into consideration.

this of course is stuff we as home reloaders cant test/see because we dont have tens of thousands of dollars in equipment - but its stuff we have to consider.

so how does buffer weight effect this? adjusting weight slows things down or speeds things up by just that fractionally marginal time (and speed) needed to let the system operate properly. we have to run fast enough but not too fast. its not a drag race. stuff happening too fast (too early) can cause failures in the system operation just like they can happening too slow (too late).

specifically relating to the 450 bushmaster - we know with reasonable confidence that 38 grains of lil gun SHOULD be functioning. that charge will cycle with many 200gr to >250gr bullets in most guns. factory ammo is cycling properly in the OP's gun. therefore its a probable assumption that we're overgassed and running harder than necessary. probably getting some bolt bounce because the buffer is bottoming out.

we know that the op only has a 3oz buffer in play. we know many of us are running 5+ oz buffers without problems. we know the system works with factory ammo so its not likely we have a gas port blockage issue.

by the assumption that he is over gassed... throwing a little more weight behind your BCG - slow things down a fraction of a second in the BCG's acceleration curve - and suddenly your over-gassed cycling speed isnt a problem anymore.

sometimes an ounce or three is just enough to keep that buffer from bottoming out on the buffer tube and letting the spring do what its supposed to on the return - instead of getting bolt bounce and causing things to run back a whole lot quicker than they otherwise would.

the good thing by using a buffer change as the test - its a NON permanent extremely low risk mod to the system. you dont have to change anything other than the buffer. theres no risk of chipping one of the lands as you open the port up. its also relatively cheap. especailly if you can borrow one from another gun or from a friend. you can increase/decrease the weight difference and if the test still fails - you can reverse the change and look for something else.

hopefully all my rambling has helped explain a little why i'm on the diagnostic track that i am.
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Re: 240xtp mag testing

Postby Nagantguy » Sun May 02, 2021 3:22 pm

Hey guys- the feed back and ideas are very much appreciated- I’ve been busy anting potatoes- our biggest plant ever and had family health problems.
Didn’t want y’all to think that I’d blown this post off or found a solution and didn’t bother sharing.
So 38.5 of lilgun under the 240 xtp mag is by far the most accurate load I’ve yet tried on n a very easy to please rifle- and as things are unubtaimum now I’ve yet to try a heavier buffer. But my hand loads of any flavor won’t lock the bolt back still. But Hornady 250 FTX and federal 300 grain none typical factory will.
So I’ve found several very accurate handloads - and still have yet to fully test my 296 loads. And as an experienced hand with the AR platform I can’t find a mechanical reason- strongly suspect either or a heavier buffer would do the trick.
It is frustrating because since this rifle was built Ive not found anything that wouldn’t shoot at least hunting acceptably accuracy and all three deer have been recovered - two on the spot third and a bit- but like many found the FTX lacking only one exit - well core exited jacket was in the liver - that shot was just a touch over 100 yards. Deer fell after running about 40 yards into a autumn olive stand with 8 inch thorns. The two other bucks - both inside 30 yards went down at the shot and looked like a anti personal mine had gone off in their thoracic cavity- great and cool- but like a mine pieces of bullet where everywhere and lots of blood shot meat. Figure these XTP mags will deliver about the same smack but hold together a bit better . I’d feel much more comfortable if everything worked as it should. So deer season here is 7 months away - and CO elk hunt is 6 months away. So I’ll get on this and get it figured cause I’m thinking the Bushmaster may be my dark timber walking around elk rifle - as a companion to the 7mm Weatherby. Last elk I took was at about 15 yards on a narrow opening in a boulder strewn valley and think the XTP and rifle and fully capable of taking elk out to about 150 yards.
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Re: 240xtp mag testing

Postby Texas Sheepdawg » Tue May 04, 2021 3:04 am

I’m just going to add my two cents worth and then I’m gonna shut up. I appreciate you guys keeping this conversation civil. So I’ll just say this.Once upon a time, I had a bunch of different plans to make my Bushmaster made, Windham Maine factory built 20 inch upper shoot “better”. Just like you guys, I ran across many different opinions on what to do to improve the function of the 450 bushmaster. Until I had a conversation with Timothy LeGendre himself, I was prepared to needlessly sink a lot of money into something that already worked just fine. He spent many years perfecting the 450 bushmaster to what it is today. He engineered everything around a mill spec A.R. 15. What he told me was this. “if your parts are mill spec and are matched properly, for example carbine buffers with carbine buffer springs in a carbine and the A1 a to buffers and buffer springs in those buffer tubes everything should run fine”. He said that I did not need to tinker with it because he already did. Well, for the most part he was right. I never did buy those expensive buffers or fancy boat carriers. My 20 inch runs just fine. Did I change the buffer spring? Yes. I use a JP polished Rifle buffer spring with a rifle buffer that is mill spec. Did I modify the magazine? Yes but most of you have seen that video know that that had nothing to do with the actual functioning of the upper. The design of the magazine had been modified to limit the number of rounds and that’s where it started acting stupid. The point is even win Bushmaster, the company tried to modify Tim’s design, things went wrong. So, maybe that can help you guys out a little bit. If you want the link to the magazine video I’m referring to I will include it below.

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Re: 240xtp mag testing

Postby plant_one » Tue May 04, 2021 10:47 am

Texas Sheepdawg wrote: My 20 inch runs just fine. Did I change the buffer spring? Yes. I use a JP polished Rifle buffer spring with a rifle buffer that is mill spec.

an A1/A2 "mil spec" rifle length and weight buffer = 5.2 oz.
a h3 weight 5/6 position adjustable stock buffer = 5.4oz.
a carbine weight 5/6 position adjustable stock buffer = 3oz.
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