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The spring needed depends also on the platform its on. The A5 buffer tube is the same size as an AR10 carbine sized buffer tube. In an A5 buffer tube you can use .223 rifle springs with the A5 buffer, or .223 extra power springs. On an AR10 type weapon you can use a 3.25" buffer with an AR10 carbine buffer spring.
You can put the spring in either way.
We can help you, but need to know exactly what is happening when it cycles. Is it short stroking? Failing to eject? Failing to feed or completely chamber? The more details the better. What was it doing before and with what parts and what is it doing now?
This has correct length for any collapsible stock AR15 with a typical carbine length buffer tube. This would be a good selection for your 10" AR and also the Sprinco Red extra power spring would also work well.
The this set up and any conventional 9mm blowback AR, use the Sprinco Orange .308 carbine spring if the barrel is 8" or more. Use the Sprinco Red extra power .223 carbine spring if the barrel is less than 8".
The 450 Bushmasters usually run best with the standard RB5000 buffer since the RB5000HP model is sometimes a bit too heavy. We normally recommend the RB5000 with a Sprinco Red Spring for the 450. If you already have the HP buffer, then you can try the Blue spring or the White spring to make up for the buffer being a bit heavy.
Not sure what strength the spring is that you have, but you should step down in power on the buffer spring. Sprinco .223 Carbine Springs in order of strongest to weakest are Red (Extra Power), Blue (Enhanced Power), White (Standard Power), Yellow (Reduced Power). Step down at least one level in spring strength.
There is some variation in what is optimal depending on the gas system of the gun. But in general, a Sprinco Red (extra power) buffer spring is good for the 6.5 Grendel and depending on how hard it cycles and ejects you can either use the RB5000 standard hydraulic model Kynshot (4.2oz) or if it ejects hard you can go a bit heavier on the buffer with an RB5000HP (4.8oz) hydraulic buffer.
It depends which buffer you use, but generally a Sprinco Red extra power carbine spring would be best.
Use the Sprinco Red extra power carbine buffer spring. Recommend using with a Kynshot RB5000 hydraulic carbine buffer as well for the 450 Bushmaster.