Consensus on bore diameter for 308/7.62x51 build

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polydeuces
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Consensus on bore diameter for 308/7.62x51 build

Post by polydeuces »

Trying to get consensus on bore diameter for 308/7.62x51 build achieving highest accuracy.
Looking for input from those with actual experience doing this.

Trying to tweak it so the utmost accuracy is achieved, which currently not impressive.
Understanding a lot of factors come to play.

Here is my current set up - running solid:
10” tube (carbon steel) mounted on Griffin Taper mount Stealth (3-prong) flash suppressor (not comp)
4” blast chamber, stainless spacer.
Baffles 6061 Alu
Baffle 1-4 bore 21/64"
Baffle 5-6 bore 11/32"
"K-cup" design
Endcap bore 3/8”

While sound suppression with this set up is impressive - mounted on a 13.5" barrel (308 gas-gun) I can 'comfortably' shoot ears open indoor - like I said accuracy is lacking, would like to change this.
Which ones to open, and how far? Whats your experience.

Really appreciate your input.
Thanks!
Last edited by polydeuces on Sun Nov 26, 2017 2:11 pm, edited 1 time in total.
Tack52
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by Tack52 »

"Accuracy is lacking" is kind of vague. How does the rifle shoot unsuppressed? What are the group sizes at what range? Are you using the same ammunition suppressed vs unsuppressed? If you want a tack driver with a can, it needs to be a tack driver without one as well
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by garredondojr »

are your baffles clipped?
partsguy22
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by partsguy22 »

21/64 (0.328) is tighter than I'd feel comfortable with I shoot for .040 over projectile diameter minimum, on my .308 can I went .375

I know that to small of a bore can negativity accuracy
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T-Rex
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by T-Rex »

21/64"? I wouldn't go under 9mm (.354in)
Going so tight, in an effort to contain the gases, can add negative influence on the projectile.
First, I would recommend opening up all your baffles.
The, give it a go and see.

Also, what type of baffle design are you using?
Any additional features that should be noted?
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CMV
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by CMV »

I used a "U" size drill. .368" As quiet as a commercial can, accuracy is not changed.
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polydeuces
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by polydeuces »

Finally have time to revisit this. Apologies for delay.
Appreciate the replies. Definitely helpful.
Without going into great detail, the poi shift across the board was well over 6", with extreme variations switching ammo (meaning significant poi varies between ammo).

Clearly too tight is just that.

Some answers on replies
It is an AR308 (gas-gun) 18" barrel, capable of sub-moa performance (problem is the indian - not the arrow)
'K' cups not clipped.
Basic design - nothing special.

Will get .368 U drill and open up diameter.
Post result (before/after) when done.
Thanks all.
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fishman
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by fishman »

Non clipped k cups? Sounds like a recipe for poor sound performance.

Clipped ks will are great for suppression, but bad for poi shift.

My ks are .375 and they are very quiet. Open those suckers up.
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yondering
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by yondering »

A couple points here:

- Just guessing from your posts, it sounds like you're drilling baffles by hand or in a drill press, not boring on a lathe, meaning concentricity is unknown and probably not great.

- Non-concentric bore holes need to be larger than concentric holes to maintain accuracy. 3/8" is probably a good place to start for all of your baffles.

- Why non-clipped K baffles? Seems like the worst possible baffle for your application, with lots of wasted space; those are intended to be clipped and used with handgun calibers (at relatively lower muzzle exit pressure and volume than your 308). Maybe consider cutting the flat face off the Ks and just use the remaining cones, add spacers in between of course.
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fishman
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by fishman »

yondering wrote:Maybe consider cutting the flat face off the Ks and just use the remaining cones, add spacers in between of course.
Are you really suggesting he commit a felony?
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by T-Rex »

fishman wrote:Are you really suggesting he commit a felony?
You're assuming he's in the US and doesn't hold or work under a manufacturing license? :wink: :wink:
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polydeuces
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by polydeuces »

Follow up:
Went ahead and opened up the baffles to 9/32 OOPS..EDIT 11/32
Switched to radial coned end cap again at 3/8
Other than that all remained the same.
(Still un-clipped)

Missing from OP specs:
Tube diameter, 1.5", carbon steel;
Spacer (blast chamber) Stainless;
Baffles 6061 Alu.

Safe to say the change was nothing short of dramatic.
Instead of the 'more than a foot and then some to left somewhere' shift, POI shift is now consistent, ~1.5" down 45˚ right.
(18" barrel, 175gr Sierra MK at 100 yds)

Another observation: I expected the blast baffle showing most wear, however it really seems the end baffle (before the end cap) taking the brunt of the abuse showing significantly more wear.
Any thoughts?

Appreciate your valued input.

Next will determine accuracy change with and w/ out can.
And, having established accuracy and sound suppression interested to see how well the 6061 baffles will hold up to a more robust firing schedule, not having been subjected to any real hard use.

Also -
I have images showing all this and more but unclear how to post.
Suggestions?
Last edited by polydeuces on Sun Dec 03, 2017 12:57 pm, edited 1 time in total.
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by fishman »

To post a picture:

Crop or dehance the picture to 1600x1600 or smaller. I use ms paint.
Upload the pic to imgur.com
Within imgur, Right click on the image and open in new tab.
On the new tab, copy the web address at the top of your browser.
Click the "img" button when writing a post on silencer talk.
Paste the link between [ img ] and [ /img ]

Here's an example. If you reply to this post you'll see the code I used to post this image.

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Re: Consensus on bore diameter for 308/7.62x51 build

Post by ECCO Machine »

polydeuces wrote:Follow up:
Went ahead and opened up the baffles to 9/32
How did you "open them up" from .328" to .281"? Was that supposed to be 11/32?

Personally, I like to be at least .030" over. I drill for .22/5.56 with a 1/4" twist drill. For .244-.284 caliber, I use 5/16", and 3/8" for .30-.338 cal. For .45 pistol cans, 1/2". Remember that all of your holes are gonna be larger than the nominal size of the drill by a few thou, too. Depending on depth of the hole and your tooling & set-up, it can be anywhere from a couple thou to >.010". Drilling a 5"-7" monocore with a 1/4" twist drill, I get a hole .255"-.258", which is fine, gives me my >.030". When we need exact size, we drill undersize & ream or use boring bars, not twist drills.
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by Hannibalbarca »

fishman wrote:Non clipped k cups? Sounds like a recipe for poor sound performance.

Clipped ks will are great for suppression, but bad for poi shift.

My ks are .375 and they are very quiet. Open those suckers up.
Speaking of clipping what about drilling holes in the baffles? This is for 60 degree cones. Say drill them at an angle or near the base of the cone. You'd have gasses going through these holes at much higher speeds than the main bore hole and they would be disrupting those gas flows. I'd also imagine if lowers the minimum required blast chamber size.
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by ECCO Machine »

Hannibalbarca wrote: Speaking of clipping what about drilling holes in the baffles? This is for 60 degree cones. Say drill them at an angle or near the base of the cone. You'd have gasses going through these holes at much higher speeds than the main bore hole and they would be disrupting those gas flows. I'd also imagine if lowers the minimum required blast chamber size.
It really depends on the overall design, but clipping or putting a radiused cut at the bore hole is most common for a reason. I've tried drilling peripheral holes in Ks, and I've put cuts down the side of monocores to accomplish the same thing. In my experience, it doesn't work as well.
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polydeuces
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by polydeuces »

Need to correct previous update - massive typo - thanks for pointing it out:

Stack is opened to 11/32.
End cap (conical) is 3/8

Apologies.

Also need to clarify baffle design.
What I called 'K" perhaps incorrect - these are identical to the ones found as storage cups for maglite tubes.
It seemed close to K but there are clear differences. Perhaps go with "Hybrid-K"?
Images forthcoming.
Any-whoo - it is more accurate (epic more) and just as quiet.

What I'm starting to realize is bore diameter affects accuracy exponentially more than sound suppression.
Which of course is a gross generalization - but apparently for this design its the case.

Appreciate all the input and tutorial on posting images - sticky worthy!
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by ECCO Machine »

polydeuces wrote:
What I'm starting to realize is bore diameter affects accuracy exponentially more than sound suppression.
Which of course is a gross generalization - but apparently for this design its the case.
Not just this case.

The through hole in most .30 cal suppressors is around .360". If you get too tight, as mentioned above, the gasses deflecting off of baffles will have more impact on bullet flight. Think of it the same as an adjustable water nozzle; the tighter you bring it in, the more any imperfections or tiny variation in concentricity of the pintle to the cone will affect the flow out the front. Well, if that pintle weren't fixed, that asymmetric pressure would push it over, as fluid dynamics tells us that flow, both liquid and gas, will try to find the path of least resistance, which means even around a radius. Since any effective suppressor is going to have asymmetry in the baffles, and since tolerances dictate that no can is going to be 100% perfectly concentric and axially aligned to the bullet's flight path, the pressure trying to escape evenly through the hole is going to act on the bullet. That's why cans change POI, and that's why insufficient clearance between bullet and through bore can (and usually does) affect accuracy. It's also why the best precision suppressors use specialized mounting systems to have the most consistency possible when removed and reinstalled.
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polydeuces
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Re: Consensus on bore diameter for 308/7.62x51 build

Post by polydeuces »

Appreciate that lesson, very helpful.
This so far has been a very gratifying educational experience, made possible by the wealth of expert knowledge available here.
Thanks all for sharing.
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