In our previous work (R.J. Asaro, P. Krysl and B. Kad, Philos. Mag. Lett. 83 (2003) p.733; P. Gu, B. Kad and M. Dao, Scr. Mater. 62 (2010) p.361), the intra-granular partial dislocation extension model was shown to be consistent with the experimental data of flow stress in nanocrystalline FCC materials. However, since the averaged extension was taken for a non-uniform loop, the model predicted small dislocation extension across the grain. In this article, extending our previous work, we reformulate the intra-granular partial dislocation model for FCC nanocrystalline materials using a more realistic loop. The flow stress obtained from the reformulated model shows good agreement with experimental data for various nanocrystalline FCC materials and expectedly large dislocation extension across the grain. In the second portion of this article, the mechanistic model for partial dislocation extension is extended to develop an intra-granular perfect dislocation extension model. The perfect dislocation model is examined by comparing its prediction of flow stress with experimental data of nanocrystalline Fe. Additionally, activation volume and strain-rate sensitivity are discussed within the mechanistic model in the light of available experimental data on nanocrystalline Fe.