Recommendations for additional parts I should look at?
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I have a few MkIV builds. I have the mamba 4.5" and it is awesome. A complete tack driver. I do have the voltquartsen competition bolt that I got on Ebay for 175. I also did the sure feed followers. For the lower I have two tandemkross Kraken complete lowers and 1 stock lower with the volquartsen accurizing kit. If you keep the stock ruger bolt the tandemkross charging loop works well...I have that on my third build. I am unsure what a tandemkross cthulu lower is...I will go look that up. EDIT. apparently that is the new name for the Kraken lowers that I have. LOL